Respond with null data when no biometrics.

Safety Center expects a response to the broadcast in this case.
The Settings app should respond with explicitely no data to Safety
Center rather than not respond.

Bug: 295143688
Test: atest BiometricsSafetySourceTest
Change-Id: Iea7f7c53919d90bf3d7fd989fb2f320a5c9ee9c6