Попробуйте так:
<xsl:if test=".//shop_group[@id=$group]/property_value[tag_name='opis']/value != ''">
<div class="bottom-group-descr">
<xsl:value-of disable-output-escaping="yes" select=".//shop_group[@id=$group]/property_value[tag_name='opis']/value" />
</div>
</xsl:if>