[xsl] how to assign a NULL to a variable?

HI, guru
Do you know how to assing a NULL to variable in XSL1.0?  Thank you.

My problem is I want write a code as follows, but I do not know how to set 
a NULL value at the following "otherwise" case:
<xsl:variable name="myvar">
  <xsl:choose>
    <xsl:when test="conditon a">
         <xsl:value-of selcet="xxx"/>
    </xsl:when>
    <xsl:when test="conditon b">
         <xsl:value-of selcet="xxx"/>
    </xsl:when>
    <xsl:otherwise>
<!--       WANT TO return a NULL value here!!!!!  But It return a "" not 
NULL -->
    </xsl:otherwise>
  </xsl:choose>
</xsl:variable>

<xsl:if test="$myvar">
.....
</xsl:if>

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