According to the new wasm-gc spec, ref.cast should forward a null input
without trapping.

Bug: v8:7748
Change-Id: Ifee17f02a572e7028c14482bc94f0e1c7fc82a5b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2647261
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72358}