br: handle region leader miss (#52822)

[ti-chi-bot]

This is an automated cherry-pick of #52822

What problem does this PR solve?

Issue Number: close #51124 close #50501

Problem Summary:
the leader of region, scanned from PD, is nil

What changed and how does it work?

check regions' leader after scan from PD.

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

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lance6716, Leavrth

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• OWNERS [Leavrth,lance6716]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-11-05 06:03:34.257881828 +0000 UTC m=+934527.097037366: ☑️ agreed by Leavrth.
• 2024-11-05 06:17:02.833927906 +0000 UTC m=+935335.673083450: ☑️ agreed by lance6716.

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 12 lines in your changes missing coverage. Please review.

Please upload report for BASE (release-7.1@5e7a36d). Learn more about missing BASE report.

Additional details and impacted files

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##             release-7.1     #52854   +/-   ##
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