br: It shows "backup failed" when make a backup of empty DB, but the result $? is 0

[joccau]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

(1) Create a database without table
(2) do the command:
./br backup db --db emptydb --pd "$ip:$port" --storage "local://$localDir"

2. What did you expect to see? (Required)

the result of "Database backup success summary" is expected and $? is 0

3. What did you see instead (Required)

[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]

4. What is your TiDB version? (Required)

./br --version
Release Version: v5.2.0-alpha-696-g72be8ee
Git Commit Hash: 72be8ee
Git Branch: fix-retry-count
Go Version: go1.16.5
UTC Build Time: 2021-08-20 13:22:07
Race Enabled: false

[joccau]

/sig migrate

[joccau]

/component br

[ti-srebot]

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