toJson method Null safety

Implementation

Map toJson() {
  Map data = Map();
  data.setValueWithOutNull(
      "paramType", chatSilentModeParamTypeToInt(_paramType));
  data.setValueWithOutNull("remindType", chatPushRemindTypeToInt(remindType));
  data.setValueWithOutNull("startTime", startTime?.toJson());
  data.setValueWithOutNull("endTime", endTime?.toJson());
  data.setValueWithOutNull("duration", silentDuration);
  return data;
}