Avoid NotImplementedError awaiting a Future[Nothing]

`gen.mkZero(NothingTpe)` gives the tree `Predef.???`. Instead, we should leave
 the `await` field uninitialized with `ValDef(..., rhs = EmptyTree)`.

Fixes #66
Merge pull request #75 from retronym/ticket/66

Avoid NotImplementedError awaiting a Future[Nothing]