Allow has_post_format() to be used to check for any format.

Calling has_post_format() with an empty list of $format will check if the
provided post has any associated format at all.

Props obenland, DrewAPicture, nacin. Fixes #24906.


git-svn-id: https://develop.svn.wordpress.org/trunk@26350 602fd350-edb4-49c9-b593-d223f7449a82