Ensure you can actually save with error_file_path=None

[TomWagg]

This pull request makes a small but important improvement to how the error_file_path attribute is handled during saving and loading in cogsworth/pop.py. The change ensures that a None value for error_file_path is consistently represented and interpreted, preventing potential issues with serialization and deserialization.

• Saving and loading logic:
  • When saving, error_file_path is stored as the string "None" if its value is None. When loading, this string is converted back to None, ensuring consistent handling of the attribute. [1] [2]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7397deb) to head (c4c7cd1).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #190   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           17        17           
  Lines         2254      2255    +1     
=========================================
+ Hits          2254      2255    +1     

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