NSE: Handle m.dummy to-device Events

[ismailgulek]

m.dummy to-device events cause NSE to crash with:

#0	0x00000001039b1b0c in Swift runtime failure: Unexpectedly found nil while implicitly unwrapping an Optional value at MatrixSDK/Background/MXBackgroundSyncService.swift:509
#1	0x00000001039b1b0c in MXBackgroundSyncService.handleToDeviceEvent(_:) at MatrixSDK/Background/MXBackgroundSyncService.swift:509
#2	0x00000001039b0fb4 in MXBackgroundSyncService.handleToDeviceEvent(_:) at MatrixSDK/Background/MXBackgroundSyncService.swift:509
#3	0x00000001039afb84 in MXBackgroundSyncService.handleSyncResponse(_:) at MatrixSDK/Background/MXBackgroundSyncService.swift:449
#4	0x00000001039acd14 in closure #2 in MXBackgroundSyncService.launchBackgroundSync(forEventId:roomId:completion:) at MatrixSDK/Background/MXBackgroundSyncService.swift:298
#5	(null) in partial apply for closure #2 in MXBackgroundSyncService.launchBackgroundSync(forEventId:roomId:completion:) ()
#6	(null) in thunk for @escaping @callee_guaranteed (@guaranteed MXResponse<MXRoomJoinRule>) -> () ()
#7	(null) in partial apply for thunk for @escaping @callee_guaranteed (@guaranteed MXResponse<MXRoomJoinRule>) -> () ()
#8	(null) in specialized closure #1 in currySuccess<A, B>(transform:_:) ()
#9	(null) in thunk for @escaping @callee_guaranteed (@in_guaranteed MXCredentials?) -> () ()
#10	(null) in partial apply for thunk for @escaping @callee_guaranteed (@in_guaranteed MXCredentials?) -> () ()
#11	(null) in thunk for @escaping @callee_guaranteed (@guaranteed NSProgress?) -> () ()
#12	(null) in _dispatch_call_block_and_release ()
#13	(null) in _dispatch_client_callout ()
#14	(null) in _dispatch_lane_serial_drain ()
#15	(null) in _dispatch_lane_invoke ()
#16	(null) in _dispatch_workloop_worker_thread ()
#17	(null) in _pthread_wqthread ()
#18	(null) in start_wqthread ()