Early return in downloadManagerViewControllerDidOpenInDriveApp

There are evidence that uploadTask can be null after some backgrounding
foregrounding of the app.
While this could not be reproduced, early return until reproduction and
fix are found.

Bug: 324897399
Change-Id: I77a71825cd2f9dda9a30745f5a1a57786b819f35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5771472
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1339505}