Question

I don’t want to in the output document to write out the element attribute if the value is missing or is an empty string.
How to do that?
That’s in a biztalk mapping.

Answer 1

If you prefer doing the XSLT yourself:
(I’m checking for missing element, empty value, and xsi:nil – delete accordingly if not applicable)

<xsl:choose>
    <xsl:when test="not(s0:inElement) 
                    or s0:inElement[normalize-space(.) = ''] 
                    or string(s0:inElement/@xsi:nil) = 'true'">
        ... Default here, e.g. leave this blank, 
            ... or if you want nil then <ns1:outElement xsi:nil="true"/>
    </xsl:when>
    <xsl:otherwise>
        <ns1:outElement>
            <xsl:value-of select="s0:inElement/text()" />
        </ns1:outElement>
    </xsl:otherwise>
</xsl:choose>