Test if the metakey really holds values and avoid PHP Warnings

git-svn-id: https://plugins.svn.wordpress.org/related/trunk@1012159 b8457f37-d9ea-0310-8a92-e5e31aec5664
Stable 1.5.6

git-svn-id: https://plugins.svn.wordpress.org/related/trunk@1012164 b8457f37-d9ea-0310-8a92-e5e31aec5664