Fix dtype error when importing groups from csv

[AnniekStok]

convert None to False for group columns so that they have a correct dtype at import.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.05%. Comparing base (d5b88b3) to head (d525066).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/funtracks/import_export/csv/_export.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #186      +/-   ##
==========================================
- Coverage   91.07%   91.05%   -0.02%     
==========================================
  Files          59       59              
  Lines        2923     2928       +5     
==========================================
+ Hits         2662     2666       +4     
- Misses        261      262       +1     

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