[BUG]: allow None for min_open_files_limit trait

[Zsailer]

The default value for this min_open_files_limit is None on Windows, but this trait doesn't (currently) allow None. This was causing issues in nbclassic unit tests, jupyter/nbclassic#63

[codecov-commenter]

Codecov Report

Merging #587 (5d10ca4) into master (ac93dd8) will decrease coverage by 0.15%.
The diff coverage is n/a.

❗ Current head 5d10ca4 differs from pull request most recent head 9b6a7e2. Consider uploading reports for the commit 9b6a7e2 to get more accurate results

@@            Coverage Diff             @@
##           master     #587      +/-   ##
==========================================
- Coverage   77.47%   77.32%   -0.16%     
==========================================
  Files         110      110              
  Lines       10212    10212              
  Branches     1104     1252     +148     
==========================================
- Hits         7912     7896      -16     
- Misses       1910     1923      +13     
- Partials      390      393       +3     

Impacted Files	Coverage Δ
jupyter_server/serverapp.py	65.44% <ø> (ø)
...yter_server/tests/services/kernelspecs/test_api.py	96.55% <0.00%> (-3.45%)	⬇️
jupyter_server/services/kernels/kernelmanager.py	79.79% <0.00%> (-2.06%)	⬇️
jupyter_server/tests/services/kernels/test_api.py	96.93% <0.00%> (-2.05%)	⬇️
jupyter_server/services/kernels/handlers.py	60.92% <0.00%> (-1.55%)	⬇️

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[kevin-bates]

LGTM - thanks @Zsailer. (I didn't try this on a Windows machine but trust traitlets are working as expected.)

[Zsailer]

Thanks, @kevin-bates!