Add a missing check for null pointer during closing of PBAP Service

This fixes NullPointerException when processing MSG_SERVERSESSION_CLOSE
from the Obex Server if Bluetooth itself is shutting down.

BluetoothPbapService.closeService() ->
BluetoothPbapObexServer.onClose() -> msg(MSG_SERVERSESSION_CLOSE) -> ...
-> BluetoothPbapService.stopObexServerSession()
-> mAdapter.isEnabled() -> NPE (mAdapter is null)

Bug: 62073087
Test: Code compilation
Change-Id: Ica98e1b784b3a51456c7709062074ea122eb8bdd