message: fix external hash join segv when build side is empty
diff: @@ -1100,6 +1100,11 @@ void JoinHashTable::UnFinalize() {
  }
  void JoinHashTable::FinalizeExternal() {
  + if (partition_block_collections.empty()) {
  + // Empty HT!
  + return;
  + }
  +
  if (finalized) {
  UnFinalize();
  }
  