backup: continue to seek regions if one range has no located leader region (#17169)

[ti-chi-bot]

This is an automated cherry-pick of #17169

What is changed and how it works?

Issue Number: Close #17168

What's Changed:

backup: continue to seek regions if one range has no located leader region

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Release note

None

[ti-chi-bot]

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[ti-chi-bot]

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