Fix two bugs coercions for union types

Fix two bugs coercions for union types

First, the union type always made a coercion, even if none its constituent types had a coercion.

Second, the union coercion always returned undef if none of the coercions
actually coerced. This caused union types which included undef to always pass
the constraint. In other case, returning undef simply led to a bad error
message saying undef did not pass the type constraint.