Always return a non-null access token to DriveFS

DriveFS expects this struct to be non-null, so for now, always give it
a value. The value itself doesn't matter as it is not used on error.

Bug: b:325351184
Change-Id: I9cfbfd15029ba73185a1b5f47717a261e7fa6302
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5316796


Commit-Queue: Austin Tankiang <austinct@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1263803}