Fall back to pandas to recognise null types

[has2k1]

closes plotnine#1021

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.52%. Comparing base (e74d5cb) to head (15c3d8a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
mizani/utils.py	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
- Coverage   99.60%   99.52%   -0.08%     
==========================================
  Files          40       40              
  Lines        2547     2549       +2     
  Branches      262      263       +1     
==========================================
  Hits         2537     2537              
- Misses          0        1       +1     
- Partials       10       11       +1     

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