Handle case of new networks that still haven't created any sites to clone from.

git-svn-id: https://plugins.svn.wordpress.org/multisite-cloner/trunk@952317 b8457f37-d9ea-0310-8a92-e5e31aec5664
The plugin now works on directory-based installs.

git-svn-id: https://plugins.svn.wordpress.org/multisite-cloner/trunk@952318 b8457f37-d9ea-0310-8a92-e5e31aec5664