AttributeError: 'NoneType' object has no attribute 'split'

For some tables I got the following error: `AttributeError: 'NoneType' object has no attribute 'split'`.

What could that be?

Stacktrace:
```
Traceback (most recent call last):
File "/Users/floris.vink/Documents/professional-services-data-validator/main.py", line 6, in
sys.exit(main())
^^^^^^
File "/Users/floris.vink/Documents/professional-services-data-validator/data_validation/__main__.py", line 582, in main
validate(args)
File "/Users/floris.vink/Documents/professional-services-data-validator/data_validation/__main__.py", line 560, in validate
run(args)
File "/Users/floris.vink/Documents/professional-services-data-validator/data_validation/__main__.py", line 523, in run
config_managers = build_config_managers_from_args(args)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/floris.vink/Documents/professional-services-data-validator/data_validation/__main__.py", line 279, in build_config_managers_from_args
config_manager = build_config_from_args(args, config_manager)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/floris.vink/Documents/professional-services-data-validator/data_validation/__main__.py", line 259, in build_config_from_args
config_manager.build_column_configs(primary_keys)
File "/Users/floris.vink/Documents/professional-services-data-validator/data_validation/config_manager.py", line 518, in build_column_configs
source_table = self.get_source_ibis_calculated_table()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/floris.vink/Documents/professional-services-data-validator/data_validation/config_manager.py", line 360, in get_source_ibis_calculated_table
table = self.get_source_ibis_table()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/floris.vink/Documents/professional-services-data-validator/data_validation/config_manager.py", line 342, in get_source_ibis_table
self._source_ibis_table = clients.get_ibis_table(
^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/floris.vink/Documents/professional-services-data-validator/data_validation/clients.py", line 134, in get_ibis_table
return client.table(table_name, database=database_name, schema=schema_name)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/floris.vink/Documents/professional-services-data-validator/venv/lib/python3.11/site-packages/ibis/backends/base/sql/alchemy/__init__.py", line 519, in table
sqla_table = self._get_sqla_table(name, database=database, schema=schema)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/floris.vink/Documents/professional-services-data-validator/venv/lib/python3.11/site-packages/ibis/backends/base/sql/alchemy/__init__.py", line 422, in _get_sqla_table
table = sa.Table(
^^^^^^^^^
File "", line 2, in __new__
File "/Users/floris.vink/Documents/professional-services-data-validator/venv/lib/python3.11/site-packages/sqlalchemy/util/deprecations.py", line 375, in warned
return fn(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^
File "/Users/floris.vink/Documents/professional-services-data-validator/venv/lib/python3.11/site-packages/sqlalchemy/sql/schema.py", line 618, in __new__
with util.safe_reraise():
File "/Users/floris.vink/Documents/professional-services-data-validator/venv/lib/python3.11/site-packages/sqlalchemy/util/langhelpers.py", line 70, in __exit__
compat.raise_(
File "/Users/floris.vink/Documents/professional-services-data-validator/venv/lib/python3.11/site-packages/sqlalchemy/util/compat.py", line 211, in raise_
raise exception
File "/Users/floris.vink/Documents/professional-services-data-validator/venv/lib/python3.11/site-packages/sqlalchemy/sql/schema.py", line 614, in __new__
table._init(name, metadata, *args, **kw)
File "/Users/floris.vink/Documents/professional-services-data-validator/venv/lib/python3.11/site-packages/sqlalchemy/sql/schema.py", line 689, in _init
self._autoload(
File "/Users/floris.vink/Documents/professional-services-data-validator/venv/lib/python3.11/site-packages/sqlalchemy/sql/schema.py", line 724, in _autoload
conn_insp.reflect_table(
File "/Users/floris.vink/Documents/professional-services-data-validator/venv/lib/python3.11/site-packages/sqlalchemy/engine/reflection.py", line 807, in reflect_table
self._reflect_indexes(
File "/Users/floris.vink/Documents/professional-services-data-validator/venv/lib/python3.11/site-packages/sqlalchemy/engine/reflection.py", line 1035, in _reflect_indexes
indexes = self.get_indexes(table_name, schema)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/floris.vink/Documents/professional-services-data-validator/venv/lib/python3.11/site-packages/sqlalchemy/engine/reflection.py", line 605, in get_indexes
return self.dialect.get_indexes(
^^^^^^^^^^^^^^^^^^^^^^^^^
File "", line 2, in get_indexes
File "/Users/floris.vink/Documents/professional-services-data-validator/venv/lib/python3.11/site-packages/sqlalchemy/engine/reflection.py", line 55, in cache
ret = fn(self, con, *args, **kw)
^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/Users/floris.vink/Documents/professional-services-data-validator/venv/lib/python3.11/site-packages/sqlalchemy/dialects/postgresql/base.py", line 4447, in get_indexes
idx_keys = idx_key.split()
^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'split'
```