In the past we've used the isolate argument to signal whether we were
in unicode mode (nullptr) or not (the real isolate). This is no longer
needed, and in fact breaks no-i18n mode which always expects to have a
real isolate.

Bug: v8:10120
Change-Id: I2f848c4ff8c2ff0e9b84278cbcdf3c3670e44e58
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2081816Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66520}