If the key of the u-ext has no value it will return error so
we should just return undefined for that case.

Bug: v8:8663
Change-Id: I7446ad74c8d0609ad268385b6f4d986060424ce9
Reviewed-on: https://chromium-review.googlesource.com/c/1404194
Commit-Queue: Frank Tang <ftang@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58682}