Don't create empty backup log files

When running a backup, if the version of a cluster is dramatically increased (say via `fdbcli advanceversion`), there will be many versions for which no mutations are created. With `BACKUP_MAX_LOG_RANGES=20` and a version jump of 20 trillion versions, this can mean a million new empty log files are created, which takes a long time. The backup agents should be able to recognize that a version range contains no mutations, and avoid creating unnecessary empty log files.