Thanks for the help everyone, I've got it working great now.  Below is
the XSLT I ended up with and a few examples of the returns I get.  All
of the input was extremely helpful in figuring out the best solution.

XSLT Change:

<!-- dc.publication fields to dc.identifier -->
<xsl:for-each select="doc:metadata/doc:element[@name='dc']/doc:element[@name='publication']">
    <dc:identifier><xsl:value-of
select="doc:element[@name='name']/doc:element/doc:field[@name='value']"/>
        <xsl:if
test="string(doc:element[@name='volume']/doc:element/doc:field[@name='value'])">
            <xsl:text> Vol. </xsl:text>
            <xsl:value-of
select="doc:element[@name='volume']/doc:element/doc:field[@name='value']"/>
        </xsl:if>
        <xsl:if
test="string(doc:element[@name='issue']/doc:element/doc:field[@name='value'])">
            <xsl:text> Issue </xsl:text>
            <xsl:value-of
select="doc:element[@name='issue']/doc:element/doc:field[@name='value']"/>
        </xsl:if>
    </dc:identifier>
</xsl:for-each>


Output Examples:

<dc:identifier>Advances in Computer Science and Engineering Vol. 1
Issue 1</dc:identifier>
<dc:identifier>Journal of Intelligent and Robotic Systems Vol.
24</dc:identifier>
<dc:identifier>Vol. 2</dc:identifier>

On Wed, Jun 3, 2015 at 3:15 PM, Matt Sherman <[log in to unmask]> wrote:
> Thanks for all the suggestions folks, other things at work have
> prevented me from working on this quite yet but there is a lot of
> helpful advice and suggestions here so thanks.  As a note on the
> <xsl:for-each> that is actually the default dc_oai.xsl file for DSpace
> and not my own work so feel free to ding the folks over there for that
> choice.  This gives me a better feel for how XSLT hands conditionals
> and some ideas on how to tackle it.  Though I would be curious to ask
> if there is a reason to default to <xsl:if> vs. <xsl:choose>?  I'm not
> sure that I necessarily need a switch to fix this problem but I want
> to hear the thought process so I know how to better think over these
> options in the future.
>
> On Tue, Jun 2, 2015 at 5:21 PM, Boheemen, Peter van
> <[log in to unmask]> wrote:
>> You should use a template that is only applied when the specified field is there. These templates in xslt are applied automatically only if the field is there:
>>
>> <xsl:template match="doc:metadata/doc:element[@name='dc']/doc:element[@name='publication']/doc:element[@name='volume']/doc:element/doc:field[@name='value']">
>>   <xsl:text>Vol. </xsl:text
>>  <xsl:apply-templates/>
>> </xsl:template>
>>
>> If the field is defined, but empty you should do:
>>
>> <xsl:template match="doc:metadata/doc:element[@name='dc']/doc:element[@name='publication']/doc:element[@name='volume']/doc:element/doc:field[@name='value']">
>>   <xsl:if test="not(.='')">
>>      <xsl:text>Vol. </xsl:text
>>      <xsl:apply-templates/>
>>   </xsl:if>
>> </xsl:template>
>>
>> Xslt is not a procedural language, you should hardly ever use <xsl:for-each>
>>
>> Peter
>>
>>
>> -----Original Message-----
>> From: Code for Libraries [mailto:[log in to unmask]] On Behalf Of Matt Sherman
>> Sent: dinsdag 2 juni 2015 21:35
>> To: [log in to unmask]
>> Subject: Re: [CODE4LIB] XSLT Advice
>>
>> Cool.  I talked to Ron via phone so I am getting a better picture, but I am still happy to take more insights.
>>
>> So the larger context.  I inherited a DSpace instance with three custom metadata fields which actually have some useful publication information, though they improperly titled them in by associating them with a dc prefix but there were two many to fix quickly and they haven't broken DSpace yet so we continue.  So I added to the XSL to pull the data within the the custom fields to display "publication name" Vol. "publication volume" Issue "publication issue".  That worked really well until I realized that there was no conditional so even when the fields are empty I still get: <dc:identifier>Vol.
>> Issue</dc:identifier>
>>
>> So here are the Custom Metadata fields:
>>
>> dc.publication.issue
>> dc.publication.name
>> dc.publication.volume
>>
>>
>> Here is the customized XSLT, with dc.identifier added for context of what the rest of the sheet looks like.
>>
>> <!-- dc.identifier -->
>>             <xsl:for-each
>> select="doc:metadata/doc:element[@name='dc']/doc:element[@name='identifier']/doc:element/doc:field[@name='value']">
>>                 <dc:identifier><xsl:value-of select="." /></dc:identifier>
>>             </xsl:for-each>
>>
>> <!-- dc.identifier.* -->
>> <xsl:for-each select="doc:metadata/doc:element[@name='dc']/doc:element[@name='identifier']/doc:element/doc:element/doc:field[@name='value']">
>>                 <dc:identifier><xsl:value-of select="." /></dc:identifier> </xsl:for-each>
>>
>> <!-- dc.publication fields to dc.identifier --> <dc:identifier><xsl:value-of select="doc:metadata/doc:element[@name='dc']/doc:element[@name='publication']/doc:element[@name='name']/doc:element/doc:field[@name='value']"/><xsl:text>
>> Vol. </xsl:text><xsl:value-of
>> select="doc:metadata/doc:element[@name='dc']/doc:element[@name='publication']/doc:element[@name='volume']/doc:element/doc:field[@name='value']"/><xsl:text>
>> Issue </xsl:text><xsl:value-of
>> select="doc:metadata/doc:element[@name='dc']/doc:element[@name='publication']/doc:element[@name='issue']/doc:element/doc:field[@name='value']"/></dc:identifier>
>>
>>
>> Ron suggested that using choose and when and that does seem to make the most sense.  The other trickiness is that I have found that some of these fields as filled when others are blank, such as their being a volume but not an issue.  So I need to figure out how to test multiple fields so that I can have it display differently dependent on what has data or not at all none of the fields are filled, which is the case in items such as posters.
>>
>> So any thoughts would help.  Thanks.
>>
>> On Tue, Jun 2, 2015 at 2:50 PM, Wick, Ryan <[log in to unmask]> wrote:
>>> I agree with Stuart, post the example here.
>>>
>>> Or if you want more real-time chat there's always #code4lib IRC.
>>>
>>> For an XSLT resource, Dave Pawson's site is great:
>>> http://www.dpawson.co.uk/xsl/sect2/sect21.html
>>>
>>> Ryan Wick
>>>
>>> -----Original Message-----
>>> From: Code for Libraries [mailto:[log in to unmask]] On Behalf
>>> Of Stuart A. Yeates
>>> Sent: Tuesday, June 02, 2015 11:46 AM
>>> To: [log in to unmask]
>>> Subject: Re: [CODE4LIB] XSLT Advice
>>>
>>> There are a number of experienced xslt'ers here. Post your example to the group so we can all learn.
>>>
>>> Cheers
>>> Stuart
>>>
>>> On Wednesday, June 3, 2015, Matt Sherman <[log in to unmask]> wrote:
>>>
>>>> Hi all,
>>>>
>>>> I am making a few corrections on an oai_dc.xslt file for our DSpace
>>>> instance I slightly botched modifying to integrate some custom
>>>> metadata into a dc.identifier citation in the OAI-PMH harvest.  I
>>>> need to get proper conditionals so it can display and harvest the
>>>> metadata correctly and not run when there is no data in those fields.
>>>> I have a pretty good idea what I need to do, and if this were like
>>>> JavaScript or Python I could probably muddle through.  The trouble is
>>>> that I don't know the conditional syntax for XSLT quite well enough
>>>> to know what I can do and thus need to do.  Plus the online resources
>>>> for learning/referencing XSLT for this are a bit shallow for what I
>>>> need hence asking the group.  So if there is anyone who knows XSLT
>>>> really well that would be willing to talk with me for a bit to help
>>>> me work through what I need to get the syntax to work like I want I
>>>> would appreciate it.  Thanks.
>>>>
>>>> Matt Sherman
>>>>
>>>
>>>
>>> --
>>> --
>>> ...let us be heard from red core to black sky