- when checking the variants of a union type we will skip adding
the variants if we have an unspecialized polymorphic, hence our
union_type variants will be empty and have a count of 0
- so when checking if we violate the #no_nil error, if we are in the
unspecialized polymorphic case and there exists at least one variant
in the original variants then we should not raise this error
- test checks that we do not raise the error anymore, and that we still
detect the #no_nil error in the described circumstances
