fix: Mean of boolean in streaming group_by incorrectly always gave NULL

[nameexhaustion]

Fixes #15610

This also happened non-deterministically on the standard engine as it sometimes ran streaming, but that will probably change soon with #15611

[codspeed-hq]

CodSpeed Performance Report

Merging #15616 will not alter performance

_{Comparing nameexhaustion:bool-mean (f3731a4) with main (92902e6)}

Summary

✅ 22 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.26%. Comparing base (c068e76) to head (f3731a4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15616      +/-   ##
==========================================
+ Coverage   81.20%   81.26%   +0.06%     
==========================================
  Files        1371     1371              
  Lines      175822   175773      -49     
  Branches     2531     2531              
==========================================
+ Hits       142782   142848      +66     
+ Misses      32563    32450     -113     
+ Partials      477      475       -2     

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