Both the .return and .throw methods are delegated to the underlying sync
iterator. These methods are retrieved using GetMethod [1], which checks
for either null or undefined instead of only undefined.

[1] https://tc39.es/ecma262/#sec-getmethod

Bug: v8:10395
Change-Id: Ide8db4270d48783da6fd1d45a5b01f2858e2828c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2258667
Auto-Submit: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68493}