sd-varlink: ensure that "any" actually means "any but null"

sd-varlink: ensure that "any" actually means "any but null"

The new "any" type was implemented by accident that it actually meant
"any but null" – unless marked as "any?" in which case it actually meant
truly any, including null. The spec change in
https://github.com/varlink/varlink.github.io/pull/43 otoh suggested that
"any" really means anything, and "any?" apparently too.

I think the implementation in code makes more sense than the spec change
however, hence let's add some checks/tests to ensure the behaviour of
the code is made explicitly and cared for.

I will prep a spec change to make the spec follow the code on this too.

Follow-up for: #39918