Fall back to notification for for blank metadata

In case the app has set a blank string in the session metadata, we should
continue checking the other fields and notification for a non-blank one.

Bug: 274775190
Test: atest MediaDataManagerTest
Change-Id: I4db207ee41a8dcaea5cd2ef079433b657ffc566e
Merged-In: I4db207ee41a8dcaea5cd2ef079433b657ffc566e
(cherry picked from commit 19628a15)