fix: Invalid usage of drop_first in to_dummies when nulls present

[mcrumiller]

Closes #25334.

If the series has nulls, then it's possible that the first tuple group will be the null group. We must ensure if drop_first=True that we drop the first non_null group, which may be the second group. So the .skip(...) sometimes gets it wrong. This fixes the issue.

I also combined some of the unit tests with parametrization for slightly better coverage.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.16%. Comparing base (18e69ac) to head (dda79dd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25435      +/-   ##
==========================================
- Coverage   82.16%   82.16%   -0.01%     
==========================================
  Files        1728     1728              
  Lines      240095   240100       +5     
  Branches     3028     3028              
==========================================
- Hits       197270   197267       -3     
- Misses      42045    42053       +8     
  Partials      780      780              

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