Empty assignment config results in AttributeError

[chrispyles]

Traceback (most recent call last):
  File "/Users/chrispyles/opt/miniconda3/envs/otter-grader/bin/otter", line 8, in <module>
    sys.exit(cli())
  File "/Users/chrispyles/opt/miniconda3/envs/otter-grader/lib/python3.10/site-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
  File "/Users/chrispyles/opt/miniconda3/envs/otter-grader/lib/python3.10/site-packages/click/core.py", line 1078, in main
    rv = self.invoke(ctx)
  File "/Users/chrispyles/opt/miniconda3/envs/otter-grader/lib/python3.10/site-packages/click/core.py", line 1688, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Users/chrispyles/opt/miniconda3/envs/otter-grader/lib/python3.10/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/chrispyles/opt/miniconda3/envs/otter-grader/lib/python3.10/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
  File "/Users/chrispyles/opt/miniconda3/envs/otter-grader/lib/python3.10/site-packages/otter/cli.py", line 32, in wrapper
    return f(*args, **kwargs)
  File "/Users/chrispyles/opt/miniconda3/envs/otter-grader/lib/python3.10/site-packages/otter/cli.py", line 64, in assign_cli
    return assign(*args, **kwargs)
  File "/Users/chrispyles/opt/miniconda3/envs/otter-grader/lib/python3.10/site-packages/otter/assign/__init__.py", line 56, in main
    write_output_directories(assignment)
  File "/Users/chrispyles/opt/miniconda3/envs/otter-grader/lib/python3.10/site-packages/otter/assign/output.py", line 108, in write_output_directories
    transformed_nb = nb_transformer.transform_notebook(nb)
  File "/Users/chrispyles/opt/miniconda3/envs/otter-grader/lib/python3.10/site-packages/otter/assign/notebook_transformer.py", line 119, in transform_notebook
    transformed_cells = self._get_transformed_cells(nb['cells'])
  File "/Users/chrispyles/opt/miniconda3/envs/otter-grader/lib/python3.10/site-packages/otter/assign/notebook_transformer.py", line 170, in _get_transformed_cells
    if config.get("config_file"):
AttributeError: 'NoneType' object has no attribute 'get'