If want to create an if-block in a xslt-file:

<xsl:if test="expr">
0
</xsl>

In the "expr" I want to decide if a value (in antother xml-file) is
nothing. This field shall be numeric, and I want to write a zero sign
instead of ''.

How can I create a expression so the if-block get executed?
Is it something like "[MyRoot/ChildNode/MyValue]==0" ?