Ignore an empty pagename when correcting is_* for page_on_front requests....

Ignore an empty pagename when correcting is_* for page_on_front requests. Props donalmacarthur, SergeyBiryukov, filosofo.  fixes #16002

git-svn-id: https://core.svn.wordpress.org/trunk@17162 1a063a9b-81f0-0310-95a4-ce76da25c4cd