Fix semantics of a null address with BT_AUDIO_LOST

Hearing aids will send BT_AUDIO_LOST to the bt route manager with a null
device when the active device goes away. Make a fix so that this
actually disconnects the device in BluetoothRouteManager.

Change-Id: Icbd6cb57af73de27ef438f34a037b0290dca771c
Fixes: 120144501
Test: unit