%GetUndetectable() is used for testing document.all's wacky "emulates
undefined" behavior both in mjsunit tests and in test262. mjsunit
doesn't rely its [[Call]] behavior, while test262 relies on its [[Call]]
returning null [1]. So, make it return null.

[1] https://github.com/tc39/test262/blob/master/INTERPRETING.md search
for IsHTMLDDA

Bug: v8:7184
Change-Id: I93d15715303deb0a932545d919bc281f5f4d5829
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2181475
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67555}