Add decoding of ref.null as a valid argument for references in
TurboFan, LiftOff and the interpreter.

R=ahaas@chromium.org
R=jkummerow@chromium.org

Bug: chromium:10063
Change-Id: I1e2d9c76f616dacb3aa06f8b535543bdcdcf0783
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1991485
Commit-Queue: Emanuel Ziegler <ecmziegler@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65788}