Drive-by: Fix ref.is_null calling is_reference_type to typecheck its
argument (which would also allow rtts).

Bug: v8:7748
Change-Id: I2ad01d0f70ac15d37ac4cc344bd0280a7ca08073
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2264094
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68572}