XSLT condition to check if node exists

I have below sample XML in which I want to check if the node exist.

Sample XML is

<document>
<item>
    <ID>1000909090</ID>
    <flex>
        <attrGroupMany name="pageinfo">
            <row>
                <attr name="pagelength">10</attr>
                <attr name="pagewidth">20</attr>
                <attr name="pageheight">30</attr>
            </row>
            <row>
                <attr name="pagelength">10</attr>
                <attr name="pagewidth">20</attr>
            </row>
        </attrGroupMany>
    </flex>
</item>
</document>

I am using below XSLT but is not getting converted

<xsl:stylesheet xmlns:xsl="http://ift.tt/tCZ8VR" version="1.0">
<xsl:output method="xml" indent="yes" />
<xsl:template match="document">
    <CatalogItem>
        <RelationshipData>
            <xsl:for-each select="item">
                <Relationship>
                    <RelationType>PAGEDETAILSINFO</RelationType>
                    <RelatedItems>
                        <xsl:attribute name="count">
                            <xsl:value-of select="count(flex//attrGroupMany[@name = 'pageinfo']/row)"/>
                        </xsl:attribute>
                        <xsl:for-each select="flex//attrGroupMany[@name = 'pageinfo']/row">
                            <xsl:choose>
                                <xsl:when test="not(string-length(attr[@name = 'pageheight'] )) = 0">
                                    <xsl:variable name="height" select="attr[@name='pageheight']"/>
                                </xsl:when>                                                         
                                <xsl:otherwise>
                                    <xsl:variable name="height" select="'0'"/>
                                </xsl:otherwise>
                            </xsl:choose> 
                            <RelatedItem>
                                <xsl:attribute name="referenceKey">
                                    <xsl:value-of select="concat('Food_And_Bev_Allergen_MVL','-',ancestor::item/ID,'-',$height)"/>
                                </xsl:attribute>
                            </RelatedItem>
                        </xsl:for-each>
                    </RelatedItems>
                </Relationship>
            </xsl:for-each>
        </RelationshipData>
    </CatalogItem>
</xsl:template>
</xsl:stylesheet>

I think the choose condition is incorrect. can you please let me know what will be the correct condition.