Fix errors when listing db backups when none exist

[markmetcalfe]

This fixes warnings that appear in the terminal output when running tdb list with no backups or .dump totara box backups

To test this:

1. Check out this PR
2. In your .env, set TDB_BACKUP_PATH to a new empty directory (e.g. set it to TDB_BACKUP_PATH="/Users/mark/test1" and run mkdir /Users/mark/test1)
3. Run tdb list and make sure no errors are output and it outputs No (db type) backups found