[xsl] How to test if existing tag/node contains a non-whitespace value?

Assume a template matches a input tag like

<aaa>(0 or more whitespaces)</aaa>

How can I test if this node contains a non-whitespace value?
It should be something similar to:

<xsl:if test="aaa=''">
Found a value
</xsl:if>

or

<xsl:if test="normalize-space(aaa)">
Found a value
</xsl:if>

Ben













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