br: handle region leader miss

[Leavrth]

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

[tiprow]

Hi @Leavrth. Thanks for your PR.

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[Leavrth]

cc @lance6716

[codecov]

Codecov Report

Merging #52822 (a024578) into master (d4a8058) will increase coverage by 2.8201%.
Report is 25 commits behind head on master.
The diff coverage is 50.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #52822        +/-   ##
================================================
+ Coverage   72.3203%   75.1405%   +2.8201%     
================================================
  Files          1473       1500        +27     
  Lines        427448     442138     +14690     
================================================
+ Hits         309132     332225     +23093     
+ Misses        99025      88841     -10184     
- Partials      19291      21072      +1781     

Flag	Coverage Δ
integration	51.3052% <0.0000%> (?)
unit	71.1844% <50.0000%> (-0.0042%)	⬇️

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

Components	Coverage Δ
dumpling	53.9957% <ø> (ø)
parser	∅ <ø> (∅)
br	59.7682% <58.3333%> (+18.5757%)	⬆️

[Leavrth]

/retest

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@Leavrth: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

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[tiprow]

@Leavrth: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

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[YuJuncen]

/test fast_test_tiprow

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@YuJuncen: The specified target(s) for /test were not found.
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@YuJuncen: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

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/test fast_test_tiprow

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #52854.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #52855.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #52856.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-5.4: #52857.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.1: #52858.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #52859.