sorry for my probably incorrect terminology, but when I say
single- tag nodes I mean a node like <img src="..."> in HTML.

As far as the result tree is concerned, you just want an empty element - an
element with no children.

But you seem more concerned with serialization. Just specify <xsl:output
method="xml"/> or <xsl:output method="html"/> and you will get a
serialization that works. It might not be exactly how you wanted it, but it
will be equivalent.