a variable reference just evaluates to a value, just like any other
xpath expression, so you test it the same way you'd test any other
expression.
You have

<xsl:if test="@image = null ">

If you want to test the variable rather than the attribute then

<xsl:if test="$image = null ">

if you want to test against a fixed string rather than the contents of a
<null> element

<xsl:if test="$image = 'hello' ">

etc

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