Callnumber type mapping file is required although not required in the configurations for HoldingsCSVTransformer

```
2022-10-26 07:00:35,325 INFO
=======ERROR=========== transform_holdings
2022-10-26 07:00:35,325 INFO unsupported operand type(s) for /: 'PosixPath' and 'NoneType' transform_holdings
2022-10-26 07:00:35,325 INFO
=======Stack Trace=========== transform_holdings
Traceback (most recent call last):
File "venv/lib/python3.9/site-packages/folio_migration_tools/migration_tasks/holdings_csv_transformer.py", line 79, in __init__
self.load_call_number_type_map(),
File "venv/lib/python3.9/site-packages/folio_migration_tools/migration_tasks/holdings_csv_transformer.py", line 152, in load_call_number_type_map
self.folder_structure.mapping_files_folder
TypeError: unsupported operand type(s) for /: 'PosixPath' and 'NoneType'
(venv)

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