[release-19.0] Fix panic in aggregation (#15728)

[vitess-bot]

Description

This is a backport of #15728

[vitess-bot]

Review Checklist

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General

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Backward compatibility

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.47%. Comparing base (0784b65) to head (657d1ab).

Additional details and impacted files

@@               Coverage Diff                @@
##           release-19.0   #15736      +/-   ##
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+ Coverage         67.43%   67.47%   +0.03%     
================================================
  Files              1560     1560              
  Lines            192763   192763              
================================================
+ Hits             129991   130058      +67     
+ Misses            62772    62705      -67     

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