I tried:
                                              <xsl:if
test="exists($productid)">
					   <Attribute name="PRODUCTID"
type="string"><xsl:value-of
select="$productid"/></Attribute>
					</xsl:if>

but got the following error:
06:47:27.062 global (ERROR) - Uncaught Exception:
'exists' is not a valid XSLT or XPath function.

<xsl:param name="myparam" select="()" />
or <xsl:param name="myparam" select="/.." />

then this yields false:
<xsl:if test="exists($myparam)" >....

<xsl:param name="myparam" select=" '' " />
or <xsl:param name="myparam" select="0" />
or <xsl:param name="myparam"  />

then this yields true:
<xsl:if test="exists($myparam)" >....

Cheers!
-- Abel Braaksma