Address corner case in which empty audio is requested and MusicRecognitionManagerService crashes.

Behavior before this change would turn on mic indicator and then crash
(leaving the indicator on indefinitely).

Second use case: Allow clients of the API to invoke the service without
actually streaming audio. For context see:
https://b.corp.google.com/issues/184808866#comment11

BUG: 192938158
BUG: 184808866
Change-Id: I17d1b7e1a489262a034d333fc42fb1dcd45c48c1