BassClientService: Fix NullPointerException

java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.Object java.util.Map.get(java.lang.Object)' on a null object reference

Process
at com.android.bluetooth.bass_client.BassClientService.getActiveSyncedSource
at com.android.bluetooth.bass_client.BassClientStateMachine.cancelActiveSync

Also minor code style fix done by repo upload

Bug: 288002172
Test: atest BluetoothInstrumentationTests
Test: manual
Tag: #feature
Change-Id: If6430ef9a6100cb0887187572d9290499608b998