Don't default to current user for capability checks when dealing with a post without an author (post_author = 0).

Undoes [12053]. While it risks breakage, this is a far safer and saner default for these situations.

props danielbachhuber.
fixes #27020.


git-svn-id: https://develop.svn.wordpress.org/trunk@27390 602fd350-edb4-49c9-b593-d223f7449a82