Fix `bysai configure` issue when data directory is empty

If `bysai configure` configure is kicked off, and the database data directory (e.g. /var/lib/redis) doesn't contain any files, bysai will crash while trying to tar an empty directory. With this update, bysai will simply bypass taring/copying data since there isn't any.

Redis fresh installs on all distros and MongoDB on RHEL/CentOS initially have empty data directories.