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

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-8.1@4546180). Click here to learn what that means.
The diff coverage is 50.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.1     #52856   +/-   ##
================================================
  Coverage               ?   72.2715%           
================================================
  Files                  ?       1468           
  Lines                  ?     426973           
  Branches               ?          0           
================================================
  Hits                   ?     308580           
  Misses                 ?      99101           
  Partials               ?      19292           

Flag	Coverage Δ
unit	71.1416% <50.0000%> (?)

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

Components	Coverage Δ
dumpling	53.9957% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	41.1218% <0.0000%> (?)

[Leavrth]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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

The pull request process is described here

Needs approval from an approver in each of these files:

• br/OWNERS [Leavrth]
• 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:

• 2024-04-24 08:01:02.484070593 +0000 UTC m=+160819.223973505: ☑️ agreed by lance6716.
• 2024-04-24 08:16:19.284066417 +0000 UTC m=+161736.023969324: ☑️ agreed by Leavrth.