This was always the intended behavior: when --embedded-variant is not
passed, we should assume "Default". What actually happened in this
case was that we'd initialize to nullptr. This CL adds the missing
nullptr check.

Bug: v8:6666
Change-Id: I87247c6c6cfec20da86d2714732a2f6044bb9b77
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1526193
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60278}