LeAudioService: Fix race on bluetooth turning off

When stressing BT ON/OFF it might happen that service is not available
when LeAudioService is trying to get it.
In such case, let's be nice and do not throw exception

Bug: 324533075
Test: atest BluetoothLeAudioTest
Flags: Exempt, trivial fix.
Change-Id: I7f05fea4f555439f4b2df55d26d6d7b8f8e1bbe4