message: fix(cycle): return empty if not exist cyclePath
diff: @@ -571,6 +571,8 @@ std::vector<int> PathQueryHandler::cycle(int uid, int vid, bool directed,
  // }
  ans = shortestPath0(uid, vid, false, pred_set);
  int lens = ans.size();
  + if (lens == 0)
  + return ans;
  for(int i = lens - 2; i > 0; --i)
  ans.push_back(ans[i]);
  ans.push_back(uid);
  