Merge: Reject arrays of null

The type `Array[null]` is not exprimable directly, and this is a good thing because it is and useless type and is broken in the compilers (except with `--erasure` for obvious reasons). However, it was still possible to create such a buggy type with literal arrays. eg `[null, null]`.

Pull-Request: #1080
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>