Drive-by:
 * Updated nullptr to using base::Optional.
 * Remove ParameterMode use in CloneFixedArray.

Bug: v8:9708, v8:6949
Change-Id: I0a98ded0a5d25df078cccbba1385d177652d1cf4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2324242Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69120}