Finally blocks that unconditionally result in an abrupt completion
immediately are currently incorrectly returning the existing completion
value instead of undefined.

Bug: v8:10978
Change-Id: Ida2e27d9cc9711236a1fb30368bfc7213d0f7140
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2473382Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70619}