feat: Added drop_nulls option to to_dummies

[florian-klein]

Motivation

I felt like it would be very useful to have this option, instead of manually dropping resulting null columns after calling to_dummies. One advantage also is that the null column is not materialized at all.

Edit: This also fixes #19095. After reading through #19325, I'm not sure if this PR is actually worthwhile moving forward. Imho I think adding this optional parameter makes a lot of sense, but let me know if you feel differently

Example:

import polars as pl

s = pl.Series("a", [1, 2, None])

print("Keep Null Column", s.to_dummies()) # current and default behavior
print("Drop Null Column", s.to_dummies(drop_nulls=True))

Keep Null Column shape: (3, 3)
┌─────┬─────┬────────┐
│ a_1 ┆ a_2 ┆ a_null │
│ --- ┆ --- ┆ ---    │
│ u8  ┆ u8  ┆ u8     │
╞═════╪═════╪════════╡
│ 1   ┆ 0   ┆ 0      │
│ 0   ┆ 1   ┆ 0      │
│ 0   ┆ 0   ┆ 1      │
└─────┴─────┴────────┘

Drop Null Column shape: (3, 2)
┌─────┬─────┐
│ a_1 ┆ a_2 │
│ --- ┆ --- │
│ u8  ┆ u8  │
╞═════╪═════╡
│ 1   ┆ 0   │
│ 0   ┆ 1   │
│ 0   ┆ 0   │
└─────┴─────┘

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.62%. Comparing base (171c847) to head (edcbc8a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23215      +/-   ##
==========================================
+ Coverage   80.61%   80.62%   +0.01%     
==========================================
  Files        1639     1639              
  Lines      220007   220027      +20     
  Branches     2814     2814              
==========================================
+ Hits       177350   177401      +51     
+ Misses      41990    41959      -31     
  Partials      667      667              

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