message: fix: not an error for router to have zero topics, r.e. reflection api
diff: @@ -276,17 +276,20 @@ json_array pubsub_man::get_topics(const std::string& realm) const
  /* EV thread */
  auto realm_iter = m_topics.find(realm);
  - if (realm_iter == m_topics.end())
  - throw std::runtime_error("realm not found");
  - wampcc::json_array topics;
  + wampcc::json_array uris;
  - const topic_registry& topic_reg = realm_iter->second;
  + // Note that it's not an error if the realm is not found in the map; that just
  + // means no topics have yet been registered.
  - for (auto & item : topic_reg)
  - topics.push_back(item.first);
  + if (realm_iter != m_topics.end()) {
  + const topic_registry& reg = realm_iter->second;
  + uris.reserve(reg.size());
  + for (auto & item : reg)
  + uris.push_back(item.first);
  + }
  - return topics;
  + return uris;
  }
  /* Add a subscription to a managed topic. Need to sync the addition of the
  