Consider the case that uloc_toLegacyType may return nullptr while
the specified keyword value cannot be mapped to a well-formed legacy type.

Bug: chromium:966285
Change-Id: I40511c54e4835599c002f1c678121341276a4e58
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1627902
Commit-Queue: Frank Tang <ftang@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61829}