SI-8269 tolerates CaseDefs with empty bodies

In macro expansions, `CaseDef(_, _, EmptyTree)` behaves almost like
`CaseDef(_, _, Literal(Constant(())))`, with the small exception of
being typechecked against WildcardType.

In this commit I propose we make these casedefs equivalent wrt typechecking
in order to reduce confusion. (The test case is taken from a real-world
project that uses macros).