how to define a namespace-prefix for null-namespace

I have to transform an input-xml-document,
whose schema doesnt define an target-namespace.
i.e the xml-instance includes xsi:noNamespaceSchemaLocation="http://www....

In my XSLT-stylesheet I wanted to define a xmlns-prefix for that
null-namespace,
in order to be independent of coming namespace-changes.

<xsl:stylesheet version="2.0" ....  xmlns:my="">
   <xsl:template match="my:*">
...

<xsl:stylesheet version="2.0" ....  xmlns:my="#default">
   <xsl:template match="my:*">
...