Union: `meet` in `Field`: Do not raise `Uncomparable` when one arg is `Top`

[michael-schwarz]

This behavior leads to unnecessary imprecision in combination with the invariant refinement for privatizations at branches.

This was added in https://github.com/goblint/analyzer/pull/1109 but it seems we were a bit overzealous in raising this exception even if one of the arguments is Top.

This fixes some of the issues described in #1456.