Switching off a filter by parameter

I have the following snippet of XSL to filter a book collection by category:

<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:param name="categor"/>
  <xsl:template match="/">
      <xsl:for-each select="books/book">
        <xsl:if test="category=$categor">
          <!-- Show info... -->
        </xsl:if>
      </xsl:for-each>
  </xsl:template>
</xsl:stylesheet>

My question: If the categor param is zero length (i.e. no filter) then how
do I test for this and allow all books through? In other words I would like
the commented code in the snippet above to be executed if the categor param
is not filled in.

Thanks!