fix: count of null column shouldn't panic in agg context

[reswqa]

Fixes #15705.

[reswqa]

NullArray is sort of not a good kid. It doesn't follow the invariant that validity = None => null count = 0. Instead, all of its elements are null, whereas validity is always None. I'm not sure if we want to add a validity field with all zeros to it.

But anyway, this change/fix can always be viewed as a fast path.

[ritchie46]

We can put this fast path immediately on L173. Then we don't have to duplicate the branch.

[reswqa]

Oh, didn't realize we had group.len(). Updated.

[codspeed-hq]

CodSpeed Performance Report

Merging #15710 will improve performances by 10.39%

_{Comparing reswqa:null_count_gb (79a86f1) with main (e39ba62)}

Summary

⚡ 1 improvements
✅ 21 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	reswqa:null_count_gb	Change
⚡	test_boolean_min_max_agg	17.1 ms	15.5 ms	+10.39%

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 81.34%. Comparing base (70b71da) to head (31f35f0).
Report is 19 commits behind head on main.

❗ Current head 31f35f0 differs from pull request most recent head 79a86f1. Consider uploading reports for the commit 79a86f1 to get more accurate results

Files	Patch %	Lines
...-lazy/src/physical_plan/expressions/aggregation.rs	50.00%	16 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #15710    +/-   ##
========================================
  Coverage   81.33%   81.34%            
========================================
  Files        1374     1375     +1     
  Lines      176471   176572   +101     
  Branches     2544     2544            
========================================
+ Hits       143539   143635    +96     
- Misses      32450    32453     +3     
- Partials      482      484     +2     

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