message: mission: fix implicit conversation
diff: @@ -78,8 +78,9 @@ public:
  * Mission items are building blocks to assemble a mission.
  */
  struct MissionItem {
  - double latitude_deg{NAN}; /**< @brief Latitude in degrees (range: -90 to +90). */
  - double longitude_deg{NAN}; /**< @brief Longitude in degrees (range: -180 to +180). */
  + double latitude_deg{double(NAN)}; /**< @brief Latitude in degrees (range: -90 to +90). */
  + double longitude_deg{
  + double(NAN)}; /**< @brief Longitude in degrees (range: -180 to +180). */
  float relative_altitude_m{
  NAN}; /**< @brief // Altitude relative to takeoff altitude in metres. */
  float speed_m_s{
  