Safer Audio.keyFor() behavior for null strings.

Apps may be relying on the original Audio.keyFor() behavior that
returned empty strings instead of null, so restore that original
behavior.

Bug: 149776788
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: Id5858ab08dc2e1b9199b96ddd7441e36ef4c126b
1 file changed