fix: Propagate null in min_by / max_by for all-null by groups

[gautamvarmadatla]

Fixes : #26918

Previously, into_no_null_iter() was used on the nullable index result from agg_arg_min/agg_arg_max. That iterator reads raw values from data_views() without checking the validity bitmap, so null entries were treated as their underlying raw buffer value. In this case that became 0, which caused all-null groups to gather the group’s first element.

I fixed this by switching to iter() so the indices stay as Option<IdxSize> through the mapping step, and by using take_unchecked(&IdxCa), which propagates null indices to null outputs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.74%. Comparing base (3ee3f1b) to head (498c92f).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #26919   +/-   ##
=======================================
  Coverage   81.73%   81.74%           
=======================================
  Files        1806     1806           
  Lines      248701   248703    +2     
  Branches     3133     3133           
=======================================
+ Hits       203287   203298   +11     
+ Misses      44609    44600    -9     
  Partials      805      805           

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[orlp]

Thanks!