log-backup: fix checkpoint display

[YuJuncen]

What problem does this PR solve?

Issue Number: fixed #36092

Problem Summary:
Before, if there isn't any checkpoint in the global scope, br log status would display no checkpoints.

What is changed and how it works?

This PR would make us display checkpoint no matter there are some checkpoints.

Check List

Tests

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

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

Fixed a bug that caused `br log status` returns no checkpoint.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• 3pointer
• joccau

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[sre-bot]

Please follow PR Title Format:

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[3pointer]

LGTM

[joccau]

LGTM

[joccau]

/run-all-tests

[sre-bot]

Please follow PR Title Format:

• pkg [, pkg2, pkg3]: what is changed

Or if the count of mainly changed packages are more than 3, use

• *: what is changed

After you have format title, you can leave a comment /run-check_title to recheck it

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/dd1e9b3949cc87f7b1ac75a68092e9156ac19e5e

[joccau]

/run-check_title