Fix case when table.original_file_path is None

[moreaupascal56]

Hello, first thanks for the package I think this can be very useful as more and more dbt projects become matures.
There was a bug because table.original_file_path is sometimes equal to None and None.startswith does not work.
So I just added a continue if clause if the table.original_file_path is equal to None.

Error Log:

  File "/Users/user/Library/Caches/pypoetry/virtualenvs/venv/lib/python3.8/site-packages/dbt_coverage/__init__.py", line 132, in filter_catalog
    if table.original_file_path.startswith(path):
AttributeError: 'NoneType' object has no attribute 'startswith'

The fix is tested successfully locally :)

fixes #52