Rollup merge of #122800 - zachs18:nonnull-slice-is_empty, r=Amanieu

Add `NonNull::<[T]>::is_empty`.

As per rust-lang/rust#71146 (comment)

I figured this should be fine to be insta-stable (with an FCP), but I can edit if that is not desired.

r? ```@Amanieu```

Author: matthiaskrgr