WebGPU: Protect against null compilation info. compilationInfo is an optional parameter of this callback. Bug: chromium:413078308 Change-Id: I12770c566b3daad5eaf8d6c8b66e464a85da06f7 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6596988 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>