message: Give ResourceId.Null a proper return type
diff: @@ -60,7 +60,11 @@ struct ResourceId
  ResourceId &operator=(ResourceId &&) = default;
  #endif
  - DOCUMENT("A helper function that explicitly creates an empty/invalid/null :class:`ResourceId`.");
  + DOCUMENT(R"(A helper function that explicitly creates an empty/invalid/null :class:`ResourceId`.
  +
  +:return: an empty/invalid/null :class:`ResourceId`.
  +:rtype: ResourceId
  +)");
  inline static ResourceId Null() { return ResourceId(); }
  DOCUMENT("Compares two ``ResourceId`` objects for equality.");
  bool operator==(const ResourceId u) const { return id == u.id; }
  