Return 404 when querying author's posts who is not a member and has no posts on the site

fixes #20601. props yoavf, nacin, SergeyBiryukov, wonderboymusic, markjaquith.
Built from https://develop.svn.wordpress.org/trunk@27290


git-svn-id: https://core.svn.wordpress.org/trunk@27146 1a063a9b-81f0-0310-95a4-ce76da25c4cd