message: Documented behaviour of empty LandmarkMapper
diff: @@ -100,6 +100,9 @@ public:
  /**
  * @brief Converts the given landmark name to the mapped name.
  *
  + * In the case the mapper is empty (no mappings defined at all), the mapper will perform
  + * an identity mapping and return the \p landmark_name that was input.
  + *
  * @param[in] landmark_name A landmark name to convert.
  * @return The mapped landmark name if a mapping exists, an empty optional otherwise.
  */
  