message: Return None from description property is there is no open result set
  This is per the db api 2.0 specification:
diff: @@ -323,7 +323,7 @@ DuckDBPyConnection *DuckDBPyConnection::Rollback() {
  py::object DuckDBPyConnection::GetAttr(const py::str &key) {
  if (key.cast<string>() == "description") {
  if (!result) {
  - throw std::runtime_error("no open result set");
  + return py::none();
  }
  return result->Description();
  }
  