message: return nullptr instead of empty data chunk when scan is exhausted
diff: @@ -75,6 +75,9 @@ unique_ptr<DataChunk> MaterializedQueryResult::FetchRaw() {
  scan_initialized = true;
  }
  collection->Scan(scan_state, *result);
  + if (result->size() == 0) {
  + return nullptr;
  + }
  return result;
  }
  