Fix missing agency_id error

[takohei]

agency_id is not a required field.
However, an error occurred if there is no agency_id in routes.txt.
The program should work even without agency_id.

An example of data that does not work is Transports Metropolitans de Barcelona.

The error message is as follows.

2023-04-24T20:35:11     WARNING    Traceback (most recent call last):
              File "C:\..\gtfs_go_dialog.py", line 279, in execution
              "features": gtfs_parser.read_route_frequency(),
              File "C:\..\gtfs_parser\__main__.py", line 542, in read_route_frequency
              self.dataframes["routes"][["route_id", "agency_id"]],
              File "C:\PROGRA~1\QGIS3~1.28\apps\Python39\lib\site-packages\pandas\core\frame.py", line 2908, in __getitem__
              indexer = self.loc._get_listlike_indexer(key, axis=1, raise_missing=True)[1]
              File "C:\PROGRA~1\QGIS3~1.28\apps\Python39\lib\site-packages\pandas\core\indexing.py", line 1254, in _get_listlike_indexer
              self._validate_read_indexer(keyarr, indexer, axis, raise_missing=raise_missing)
              File "C:\PROGRA~1\QGIS3~1.28\apps\Python39\lib\site-packages\pandas\core\indexing.py", line 1304, in _validate_read_indexer
              raise KeyError(f"{not_found} not in index")
             KeyError: "['agency_id'] not in index"