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

[Leavrth]

/test pull-br-integration-test

[tiprow]

@Leavrth: No presubmit jobs available for pingcap/tidb@release-7.5

In response to this:

/test pull-br-integration-test

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

The full list of commands accepted by this bot can be found here.

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Needs approval from an approver in each of these files:

• br/OWNERS [YuJuncen]
• br/pkg/lightning/OWNERS [lance6716]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-05-22 07:33:32.199194248 +0000 UTC m=+2243365.956329822: ☑️ agreed by lance6716.
• 2024-05-22 09:13:55.497336271 +0000 UTC m=+2249389.254471843: ☑️ agreed by YuJuncen.

[Leavrth]

/retest

[Leavrth]

/retest

[codecov]

Codecov Report

Attention: Patch coverage is 36.36364% with 14 lines in your changes are missing coverage. Please review.

Please upload report for BASE (release-7.5@e406d57). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-7.5     #52855   +/-   ##
================================================
  Coverage               ?   71.9948%           
================================================
  Files                  ?       1411           
  Lines                  ?     410253           
  Branches               ?          0           
================================================
  Hits                   ?     295361           
  Misses                 ?      95027           
  Partials               ?      19865           

Flag	Coverage Δ
unit	71.9948% <36.3636%> (?)

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

Components	Coverage Δ
dumpling	53.9913% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	53.4049% <0.0000%> (?)