scheduler: add leader verify in balance-region scheduler (#2966)

[JmPotato]

What problem does this PR solve?

Issue Number: ref #2966.

What is changed and how does it work?

Check region leader before balance region. If it has no leader, the balance for this region would be skipped.

Check List

Tests

• Unit test
• Integration test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• HuSharp
• bufferflies

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