Fix NPE when null BackupObserver is passed into BackupManager#requestBackup()

BackupManagerService has the null check, but it doesn't work
because passed BackupObserver object is wrapped into
this BackupObserverWrapper.
This was found during GTS testing.

Bug: 27334738
Change-Id: I16dc0230824b326d3fae1f8489f58b0c0d4e1c7c