Some fields didn't have default values, which made it impossible to
check whether or not all fields are initialized.

Bug: chromium:1056170
Change-Id: I8c6f89e64661420460bfc351473a47a623ef6399
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3243462Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77535}