XSL - Check if select are NULL

How can I check if my block/select are NULL then show a text

    <xsl:choose>    
    <xsl:when test="@pass='0' or @pass='1'">
    <fo:block padding-top="1cm" font-weight="bold" margin-bottom="0.2cm">Header</fo:block>                                          <fo:block margin-top="0.2cm" margin-bottom="0.2cm" border-bottom="1px solid #e1e1e1">
</fo:block>
    <fo:block margin-top="0.2cm" margin-bottom="0.2cm" padding-left="5px" padding-right="5px" padding-top="1px">
    <xsl:apply-templates select="detail/katalog/frage[@ko='1' and @passed='0']" />
    <xsl:apply-templates select="detail/katalog/frage/options" />
    <xsl:apply-templates select="detail/katalog/matrix/frage/options" />
    <xsl:apply-templates select="detail/katalog/clusterArea/clusterFrage[@ko='1' and @passed='0']" />
    </fo:block>

I tried this, but it doesn't work.

<xsl:if test="(not(detail/katalog/frage[@ko='1' and @passed='0']) and not(detail/katalog/clusterArea/clusterFrage[@ko='1' and @passed='0']) and not(detail/katalog/frage/options) and not(detail/katalog/matrix/frage/options))">
<fo:block>
Not Results
</fo:block>
 </xsl:if>

Thanks a lot! PR