If we are near the stack limit, calling the proxy method might not work
any more. Instead of crashing because of an empty MaybeLocal, handle
this gracefully.

Drive-by: Minor refactoring in TryGetValue.

R=tebbi@chromium.org

Bug: chromium:1110001
Change-Id: I07e7773768166b3dbea2e6b75a3ab8b24bfeee53
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2332156Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69161}