executor: fix nil buffer in basePartialResult4GroupConcat when hash aggregation is spilled

[xzhangxian1008]

What problem does this PR solve?

Issue Number: close #61749

Problem Summary:

What changed and how does it work?

When all values that passed in group_concat are null, buffer in basePartialResult4GroupConcat should be nil, and we need to consider it when serializing and deserializing the basePartialResult4GroupConcat.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Fix the issue that buffer in basePartialResult4GroupConcat may be nil when hash aggregation is spilled

[tiprow]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.4398%. Comparing base (8fc1430) to head (0bc8ad1).
Report is 97 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #61808        +/-   ##
================================================
+ Coverage   73.0434%   73.4398%   +0.3963%     
================================================
  Files          1730       1730                
  Lines        481687     481948       +261     
================================================
+ Hits         351841     353942      +2101     
+ Misses       108289     106465      -1824     
+ Partials      21557      21541        -16     

Flag	Coverage Δ
integration	42.3372% <0.0000%> (?)
unit	72.2885% <100.0000%> (-0.0135%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	52.7804% <ø> (ø)
parser	∅ <ø> (∅)
br	46.5728% <ø> (-0.0105%)	⬇️

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-06-18 09:50:04.455253969 +0000 UTC m=+265257.178432952: ☑️ agreed by XuHuaiyu.
• 2025-06-18 12:35:09.801769393 +0000 UTC m=+275162.524948377: ☑️ agreed by dveeden.

[xzhangxian1008]

/run-check-issue-triage-complete

[dveeden]

/check-issue-triage-complete

[xzhangxian1008]

/hold

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dveeden, gengliqi, XuHuaiyu

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Needs approval from an approver in each of these files:

• OWNERS [XuHuaiyu,dveeden]

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[xzhangxian1008]

/unhold

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #62124.
But this PR has conflicts, please resolve them!

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #62125.
But this PR has conflicts, please resolve them!