While this field isn't used, inititialize it to null.

Bug: v8:11708
Change-Id: I9698e73183f49ef54b8978383e1406e5cf765c75
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3562982
Auto-Submit: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79710}