api: fix panic when region doesn't have a leader (#7629)

[ti-chi-bot]

This is an automated cherry-pick of #7629

What problem does this PR solve?

Issue Number: Close #7630.

What is changed and how does it work?

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lhy1024
• rleungx

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[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (71d81a5) 74.82% compared to head (6639f9d) 74.80%.
Report is 6 commits behind head on release-7.1.

Files	Patch %	Lines
server/api/region.go	68.00%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release-7.1    #7649      +/-   ##
===============================================
- Coverage        74.82%   74.80%   -0.03%     
===============================================
  Files              411      411              
  Lines            41632    41687      +55     
===============================================
+ Hits             31151    31183      +32     
- Misses            7739     7762      +23     
  Partials          2742     2742              

Flag	Coverage Δ
unittests	74.80% <69.23%> (-0.03%)	⬇️

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[rleungx]

/merge

[ti-chi-bot]

@rleungx: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 6639f9d

[ti-chi-bot]

@ti-chi-bot: Your PR was out of date, I have automatically updated it for you.

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