[Bass] Fix the usage of notifySourceAddFailed to avoid null metadata

Per the API, onSourceAddFailed requires non-null metadata.
This commit includes the improvements:
1. In addSource(), no need to invoke the callback if metadata is null. This non-null requirement is enforced in the framework API implementation.
2. For internal triggered calling addSource or notifySourceAddFailed, avoid the null metadata from callers.

Bug: 316244219
Bug: 316005152
Test: atest BassClientServiceTest BassClientStateMachineTest
Change-Id: I6e2c5e26357ca0294f1f965cabf96d9ff3a7eb97