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]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/check_dev	0bfb6b4	link	true	/test check-dev
idc-jenkins-ci-tidb/unit-test	0bfb6b4	link	true	/test unit-test

Full PR test history. Your PR dashboard.

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