scanUsbAccessory method Null safety

Implementation

@override
Future<List<UsbAccessoryDevice>?> scanUsbAccessory() async {
  return methodChannel
      .invokeMethod(PluginConstants.methodScanUsbAccessoryDevice)
      .then((value) {
    return proto.UsbAccessoryList.fromBuffer(value)
        .items
        .map((e) => UsbAccessoryDevice.fromProto(e))
        .toList();
  });
}