ddl: fix a bug that index with empty name can be created unexpectedly

[CbcWestwolf]

What problem does this PR solve?

Issue Number: close #61265

Problem Summary:

If a duplicate name index is found with IF NOT EXISTS, tidb report a warning

tidb/pkg/ddl/executor.go

Lines 4715 to 4718 in 0d8e2bb

	if ifNotExists {
	ctx.AppendNote(err)
	return ast.CIStr{}, nil, nil
	}

but do not skip the following execution

tidb/pkg/ddl/executor.go

Lines 4915 to 4920 in 0d8e2bb

	indexName, hiddenCols, err := checkIndexNameAndColumns(metaBuildCtx, t, indexName, indexPartSpecifications, model.ColumnarIndexTypeNA, ifNotExists)
	if err != nil {
	return errors.Trace(err)
	}
	tblInfo := t.Meta()

What changed and how does it work?

If a duplicate name index is found, skip the following execution.

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.9789%. Comparing base (d216b7b) to head (5b2cbd8).
Report is 7 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #61531        +/-   ##
================================================
+ Coverage   73.0963%   74.9789%   +1.8825%     
================================================
  Files          1729       1751        +22     
  Lines        480686     489395      +8709     
================================================
+ Hits         351364     366943     +15579     
+ Misses       107793      99477      -8316     
- Partials      21529      22975      +1446     

Flag	Coverage Δ
integration	48.8747% <100.0000%> (?)
unit	72.3764% <100.0000%> (+0.0302%)	⬆️

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	62.9013% <ø> (+15.9320%)	⬆️

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[CbcWestwolf]

/retest

[CbcWestwolf]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fzzf678, tangenta

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Needs approval from an approver in each of these files:

• OWNERS [tangenta]
• pkg/ddl/OWNERS [tangenta]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-06-06 05:52:25.344140947 +0000 UTC m=+593831.546486212: ☑️ agreed by tangenta.
• 2025-06-06 05:57:07.241235584 +0000 UTC m=+594113.443580847: ☑️ agreed by fzzf678.

[CbcWestwolf]

/cherry-pick release-8.5

[ti-chi-bot]

@CbcWestwolf: new pull request created to branch release-8.5: #61552.

In response to this:

/cherry-pick release-8.5

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