planner/core: fix index out of bound bug when empty dual table is remove for mpp query (#28251)

[ti-srebot]

cherry-pick #28251 to release-5.1
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/28279

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/28279:release-5.1-452d34c1ee27

---

What problem does this PR solve?

Issue Number: close #28250

Problem Summary:
As described in the issue.

What is changed and how it works?

Proposal: xxx

What's Changed:

How it Works:
The root cause is after #25218, union may contain less than 2 children, but in stringer.ToString, it still assume that union always contains at least 2 children. This pr refine the related code to consider the case that union contains less than 2 children.

Check List

Tests

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

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

Fix index out of bound bug when empty dual table is remove for mpp query

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• fzhedu
• wjhuang2016

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[ti-srebot]

/run-all-tests

[ti-srebot]

@windtalker you're already a collaborator in bot's repo.

[fzhedu]

/LGTM

[windtalker]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: cb50703

[ti-chi-bot]

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[windtalker]

/run-check_dev