Fix a notice in [20565] where get_the_author_meta() may be called prior to...

Fix a notice in [20565] where get_the_author_meta() may be called prior to postdata from being set up. In this case, the function should return nothing (via the filter). props johnjamesjacoby. fixes #20529. see #20285.

git-svn-id: https://core.svn.wordpress.org/trunk@20575 1a063a9b-81f0-0310-95a4-ce76da25c4cd