[normalizer][easy] Type.NullProto normalizes to Ty.Null
[normalizer][easy] Type.NullProto normalizes to Ty.Null

Summary:
`NullProtoT` behaves as `null` as a lower bound. The normalizer almost
exclusively accounts for lower-bounds, so this type can be normalized to `null`.

Reviewed By: jbrown215

Differential Revision: D16174234

fbshipit-source-id: ce60e55330c0c6277de0357fc6f5de07770be6b9

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow