message: mission: import null as NaN
  When QGC sets a param as `null` it means unchanged which should
  translate to NaN for mavlink mission items.
diff: @@ -1203,8 +1203,13 @@ Mission::Result MissionImpl::import_mission_items(Mission::mission_items_t &all_
  // Extract parameters of each mission item
  std::vector<double> params;
  for (auto &p : json_mission_item["params"].array_items()) {
  + if (p.is_null()) {
  + // QGC sets params as `null` if they should be unchanged.
  + params.push_back(NAN);
  + } else {
  params.push_back(p.number_value());
  }
  + }
  result = build_mission_items(command, params, new_mission_item, all_mission_items);
  if (result != Mission::Result::SUCCESS) {
  