The FastTemplateCache gets filled with undefiend when created and then
with holes when it grows causing a mismatch.

This hasn't been a problem so far as the FastTemplateCache is always
initialized to the max size but it could cause problems in the future
if this is changed.

Bug: v8:11289
Change-Id: I8dabce101b25d9f0057183c88f507873d973f2a8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2608472Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Sathya Gunasekaran  <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71939}