Bad call to is_empty() instead of empty().

git-svn-id: https://plugins.svn.wordpress.org/wpgform/trunk@944420 b8457f37-d9ea-0310-8a92-e5e31aec5664
Prep for v0.72 release.

git-svn-id: https://plugins.svn.wordpress.org/wpgform/trunk@944422 b8457f37-d9ea-0310-8a92-e5e31aec5664