backup: backup empty databases (#34385)

[ti-srebot]

cherry-pick #34385 and #34519 to release-5.4
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/35128

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/tidb.git pr/35128:release-5.4-9339955f0d84

---

What problem does this PR solve?

Issue Number: close #33866

Problem Summary:

• Point-in-time recovery need we backup empty database during full backup.

What is changed and how it works?

• This PR tried to backup empty database and add an integration test.

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

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.

Update the behaviour the br can backup empty databases.

[ti-chi-bot]

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[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-srebot]

/run-all-tests

[ti-srebot]

@3pointer you're already a collaborator in bot's repo.