promiseCapability can be undefined.

Bug: v8:11025
Bug: chromium:1201113
Change-Id: I9da8764820cee0db1f0c38ed2fff0e3afeb9a80e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2844649Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74117}