ToNumber was already returning base::Optional but it still needed to be
updated for the internal external uncached string case.

As a note, both IsExternal and IsSeqString do not need to be updated
since they only look at the map.

Bug: v8:7790
Change-Id: Icb5ba7f40982c01cada2a9c2b96b824edce70d44
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2615422Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72005}