[dfmc-modeling] Fix ^known-disjoint? crash with singletons and unions.

We can't check ^known-disjoint? with the value of a singleton as this
may not be a type and ^known-disjoint? is defined on <type> and not
<object>.

This corrects an error made in commit 64c58256781aa34e1bdbe7c39ab9bdcf09148f28.
Merge pull request #775 from waywardmonkeys/fix-known-disjoint-for-singleton-union

[dfmc-modeling] Fix ^known-disjoint? crash with singletons and unions.