fix: ewm_mean_by was skipping initial nulls when it was already sorted by "by" column

[MarcoGorelli]

On the latest release, initial null values are skipped when the dataframe is already known to be sorted by 'by', resulting in errors like

ShapeError: unable to add a column of length 4 to a DataFrame of height 5

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.37%. Comparing base (96e1f01) to head (1faf7fd).
Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15812      +/-   ##
==========================================
- Coverage   81.35%   80.37%   -0.99%     
==========================================
  Files        1379     1264     -115     
  Lines      176603   165423   -11180     
  Branches     2544        0    -2544     
==========================================
- Hits       143677   132960   -10717     
- Misses      32443    32463      +20     
+ Partials      483        0     -483     

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[codspeed-hq]

CodSpeed Performance Report

Merging #15812 will degrade performances by 12.17%

_{Comparing MarcoGorelli:fix-ewm-mean-by-sorted-null-first (1faf7fd) with main (937fd46)}

Summary

❌ 1 regressions
✅ 21 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	MarcoGorelli:fix-ewm-mean-by-sorted-null-first	Change
❌	test_sort_nan_1942	1 ms	1.2 ms	-12.17%