fix: Don't count nulls in streaming count agg

[ritchie46]

fixes #15049

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.02%. Comparing base (82932c6) to head (7366d65).
Report is 6 commits behind head on main.

❗ Current head 7366d65 differs from pull request most recent head a3447a1. Consider uploading reports for the commit a3447a1 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15051   +/-   ##
=======================================
  Coverage   81.01%   81.02%           
=======================================
  Files        1338     1338           
  Lines      173595   173593    -2     
  Branches     2461     2461           
=======================================
+ Hits       140635   140647   +12     
+ Misses      32490    32477   -13     
+ Partials      470      469    -1     

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