Fixed zfs-replicate bug when a filesystem is created on the backup drive,

and the snapshots for that filesystem are all deleted over time, and new
ones are attempted to be sent to the backup drive. In this case, the script
was trying to send incremental, but there were no snapshots left on the
zfs filesystem. Now, if a filesystem exists on the backup drive but it
has no snapshots, that filesystem is removed so a full stream will be sent
of the first snapshot that is synced to the backup drive.