br:backup: If make a backup of empty db, should get a success summary

[joccau]

What problem does this PR solve?

Issue Number: close #27466

Problem Summary:

What is changed and how it works?

What's Changed:
If make a backup for a empty database, return a successful summary result

Tests

• Manual test (add detailed scripts or steps below)

before fix it

[2021/08/21 12:29:09.666 +08:00] [INFO] [collector.go:66] ["Database backup failed summary"] [total-ranges=0] [ranges-succeed=0] [ranges-failed=0]

after fix it

[2021/08/21 13:42:24.735 +08:00] [INFO] [collector.go:65] ["Database backup success summary"] [total-ranges=0] [ranges-succeed=0] [ranges-failed=0] [total-take=27.584125ms] [BackupTS=427170082246885378]

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• 3pointer
• YuJuncen

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[joccau]

/component br

[joccau]

/build

[joccau]

/cc @3pointer
/cc @YuJuncen
/cc @fengou1
/cc @glorv

[ti-chi-bot]

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[joccau]

/run-integration-tests

[joccau]

/run-integration-tests

[kennytm]

/merge

[ti-chi-bot]

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

Commit hash: 38ff83d

[joccau]

/run-all-tests

[kennytm]

/merge

[ti-chi-bot]

@joccau: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

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