fix: Incorrectly gave list.len() for masked-out rows

[nameexhaustion]

Fixes #19987

[codspeed-hq]

CodSpeed Performance Report

Merging #19999 will improve performances by 27.62%

_{Comparing nameexhaustion:list-lengths-validity (7ef977b) with main (d9ea1d8)}

Summary

⚡ 1 improvements
✅ 40 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	nameexhaustion:list-lengths-validity	Change
⚡	test_pdsh_q18	16.5 ms	12.9 ms	+27.62%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.49%. Comparing base (0b1d520) to head (7ef977b).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19999      +/-   ##
==========================================
- Coverage   79.49%   79.49%   -0.01%     
==========================================
  Files        1556     1556              
  Lines      216407   216416       +9     
  Branches     2456     2456              
==========================================
+ Hits       172043   172044       +1     
- Misses      43806    43814       +8     
  Partials      558      558              

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