planer: fix invalid pointer caused by a recursive CTE query (#54450)

[ti-chi-bot]

This is an automated cherry-pick of #54450

What problem does this PR solve?

Issue Number: close #54449

Problem Summary:

What changed and how does it work?

in the FindBestTask, we should return base.InvalidTask when it doesn't meet requirement. but now it returns nil. it is the root causion.

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.

None

[hawkingrei]

/retest

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (release-8.1@ae394c6). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.1     #54677   +/-   ##
================================================
  Coverage               ?   71.2865%           
================================================
  Files                  ?       1465           
  Lines                  ?     422279           
  Branches               ?          0           
================================================
  Hits                   ?     301028           
  Misses                 ?     100787           
  Partials               ?      20464           

Flag	Coverage Δ
unit	71.2865% <100.0000%> (?)

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

Components	Coverage Δ
dumpling	53.9957% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	41.0591% <0.0000%> (?)

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hawkingrei, winoros

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [hawkingrei,winoros]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-07-19 07:11:11.562749835 +0000 UTC m=+597093.553691303: ☑️ agreed by hawkingrei.
• 2024-07-22 05:57:22.071357092 +0000 UTC m=+851864.062298559: ☑️ agreed by winoros.