Side product: enable null as __proto__.

Bug: v8:11525,v8:12820
Change-Id: I2b9508d0f3563d9000ddede24e7684aab18c2b5e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3637791Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80474}