[xsl] Checking if a variable has a value

["Mark Giffin m1879@xxxxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

XSLT 2.0, Saxon processor.

What's a good way to check if a variable has a value? I want to print
something if a variable has a value, and not print it if it is not given a
value. This must be extremely simple but I don't see it.

Variable is declared:
<xsl:param name="git-hash"/>

This will sometimes be given a string value like this:
26166cf7f809f5107778c2742e06edb024c5ffef

I tried this and others:
<xsl:if test="matches($git-hash, '\d|[a-z]')">
    <dc:subject>git hash: <xsl:value-of select="$git-hash"/></dc:subject>
</xsl:if>

When the variable has no value, the following is output instead of nothing:
git hash: ${git-hash}

The following works but only if the value has numbers in it:
<xsl:if test="matches($git-hash, '\d')">

None of these work, what have I missed:
<xsl:if test="$git-hash">
<xsl:if test="boolean($git-hash)">
<xsl:if test="matches($git-hash, '\d|\S')">
<xsl:if test="matches($git-hash, '\d|[a-z]')">

<xsl:choose>
   <xsl:when test="not($git-hash)"></xsl:when>
   <xsl:otherwise>
       <dc:subject>git hash: <xsl:value-of select="$git-hash"/></dc:subject>
   </xsl:otherwise>
</xsl:choose>

Thanks,
Mark