Audio: Check if BluetoothDevice.getName() returns null

* BluetoothDevice.getName() could return null if Bluetooth process
  is not running. This patch uses empty string "" to replace the null
  value returned from BluetoothDevice.getName(). The logic is wrapped
  in BtHelper#getName(BluetoothDevice) method

Fixes: 129806133
Test: atest BluetoothInstrumentationTests on eng build
Change-Id: If44e675aafa779fa86451742aa917404987fc15a