message: allow construction of empty ProguardMap
  Summary: I think there's no reason why this should not be default constructible. It makes sense for all of its field to be empty.
diff: * For fields, it's <class descriptor>.<name>:<type desc> .
  */
  struct ProguardMap {
  + /**
  + * Construct an empty ProGuard map.
  + */
  + explicit ProguardMap() = default;
  +
  /**
  * Construct map from the given file.
  */
  