[release-20.0-rc] Fix vtgate crash in group concat

[GuptaManan100]

Description

This PR only backports the vtgate crash fix from #16237.

Related Issue(s)

• Fixes Bug Report: Group_concat with separator crashes vtgate during planning #16238

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 68.53%. Comparing base (e8da04f) to head (61ba53d).

Files	Patch %	Lines
...lanbuilder/operators/aggregation_pushing_helper.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##           release-20.0-rc   #16253      +/-   ##
===================================================
+ Coverage            68.52%   68.53%   +0.01%     
===================================================
  Files                 1541     1541              
  Lines               197788   197788              
===================================================
+ Hits                135525   135550      +25     
+ Misses               62263    62238      -25     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[GrahamCampbell]

If this does hit the RC branch, doesn't this mean the 20.0.0 release is necessarily delayed, because an RC3 will be needed?

[frouioui]

If we merge this we must do an RC-3.

[systay]

I don't think we should do a new RC based on this. It's an old issue that's been in many releases in the past.

[frouioui]

I don't think we should do a new RC based on this. It's an old issue that's been in many releases in the past.

Should we close this PR then?

[systay]

If this PR signals the release of a new RC, we should close it.