The first two fixes concern characterization of SAM types. One condition of a SAM
type is that it can be instantiated with an empty argument list. This was implemented
incorrectly. First, we missed the case where the SAM type is a trait with a parent
class that takes arguments. In this case the SAM type _cannot_ be instantiated with an
empty argument list. Second, we missed the case where the SAM type constructor has a
single vararg parameter. In this case the SAM type _can_ be instantiated with an empty
argument list.

The second case was also translated incorrectly which led to illegal bytecodes.

Fixes scala#15855

Fix SAM test to use the same scheme as SAM expansion to determine whether
a type needs zero arguments for construction.