[compiler] Handle empty optional in ReadFeedbackForGlobalAccess

Alternatively, the CHECK and load could be removed.

Bug: v8:7790,chromium:1237309
Change-Id: I45b1495002a47f2f4ff2915c7997e34c79c1aed2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3080561


Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76154}