Ensure that we always coerce the class list passed to get_body_class to an...

Ensure that we always coerce the class list passed to get_body_class to an array even when it is empty. Fixes #17717

git-svn-id: https://core.svn.wordpress.org/trunk@18176 1a063a9b-81f0-0310-95a4-ce76da25c4cd