XSLT xsl:apply-templates Conditional Syntax - xslt

Question

I've got the following XSLT code which lists out the folders and their file items from a specified node.
This all works fine but I'd like to parameterise the page and optionally filter its output by a tag value.
Being an XLST numpty I'm stumped with the syntax for the conditional I should be putting in under the <xsl:when test="$tag"> clause - can someone please help ?
<xsl:variable name="tag" select="umbraco.library:Request('tag')" />
<xsl:template match="/">
<!-- Root folder in Media that holds the folders to output -->
<xsl:variable name="mediaRootFolderId" select="5948" />
<!-- Pass in true() to get XML for all nodes below -->
<xsl:variable name="mediaRootNode" select="umbraco.library:GetMedia($mediaRootFolderId, true())" />
<xsl:choose>
<xsl:when test="$tag">
</xsl:when>
<xsl:otherwise>
<!-- If we didn't get an error, output Folder elements that contain Image elements -->
<xsl:apply-templates select="$mediaRootNode[not(error)]/Folder[File]" >
<xsl:sort select="#nodeName"/>
</xsl:apply-templates>
</xsl:otherwise>
</xsl:choose>
</xsl:template>
<!-- Template for folders -->
<xsl:template match="Folder">
<div class="folder">
<h2>Folder: <xsl:value-of select="#nodeName" /></h2>
<div class="images">
<xsl:apply-templates select="File">
<xsl:sort select="#nodeName"/>
</xsl:apply-templates>
</div>
</div>
</xsl:template>
<!-- Template for files -->
<xsl:template match="File">
File: <a href="{umbracoFile}" alt="{#nodeName}" ><xsl:value-of select="#nodeName" /></a> <br/>
</xsl:template>

Answer 1

Instead of the long <xsl:choose> instruction, use:
<xsl:apply-templates select=
"$mediaRootNode[not($tag)][not(error)]
/Folder[File]" >
Explanation: For the XPath expression in the select attribute above to select a non-empty set of nodes it is necessary that boolean($tag) is true(). Thus the above single <xsl:apply-templates> instruction is equivalent to the long <xsl:choose> in the question.

Answer 2

you can test if $tag is set like this.
<xsl:param name="tag">
<xsl:message terminate="yes">
$tag has not been set
</xsl:message>
</xsl:param>
This isn't standard though, it'll work on most XSLT processors though.
If you wanted to be absolutely save, you could set the value to an illegal value (such as 1 div 0) and test for it in the body of the template:
<xsl:param name="tag" select="1 div 0" />
<xsl:if test="$tag = 1 div 0">
<xsl:message terminate="yes">
$tag has not been set, or has been set to Infinity, which is invalid.
</xsl:message>
</xsl:if>
Source: O'Reilly XSLT Cookbook