First binlog backup fails after applying #604

[Shunki]

Describe the bug
Immediately after application, lastBackup.UUIDSet is empty, so choosableIndexes is also empty.

moco/backup/backup.go

Line 253 in a7352fa

 choosableIndexes := getIdxsWithUnchangedUUID(bm.uuidSet, lastBackup.UUIDSet) 

Expected behavior
Can get binlog backups immediately after applying #604

Additional context
Add any other context about the problem here.