Comparing to SQL NULL

I am writing some XSL code processing database columns
(actually, adding a new column to the existing table in old application)
and need to handle a situation when the column does not have a value yet.
So how do I express that condition in XSL:
<xsl:when test="$v1 = 'NULL'">
... do something
</xsl:when>
or <xsl:when test="$v1 = 'NaN'">
... do something
</xsl:when>
or <xsl:when test="$v1 = ''">   ... blank string: ''
... do something
</xsl:when>  ???

Sorry for the newbie question.
Using XSLT1.0 [and Cocoon 2.0.x]

Thank you,
Oleg.