AudioService: Set BT device name as empty string when it is null

When HFP active device changes, AudioService will update new
device information in its device hash map along with the device
name. During BT turn off, getting the device name may fail or
return null. In this case, pass empty string as an argument.

CRs-Fixed: 2307153
Change-Id: I3dc2732d5ca9f597bebc5c9d8473711c64b24a94