Remove unnecessary loops in default-filters.php.

props Jayjdk.
fixes #25011.


git-svn-id: http://develop.svn.wordpress.org/trunk@25134 602fd350-edb4-49c9-b593-d223f7449a82
Only show 'Preview' on the custom header screen if there is something to show.

props nickmomrik.
fixes #14531.


git-svn-id: http://develop.svn.wordpress.org/trunk@25135 602fd350-edb4-49c9-b593-d223f7449a82
Ensure a user exists in get_comment_author().

props azaozz.
see #24084.


git-svn-id: http://develop.svn.wordpress.org/trunk@25136 602fd350-edb4-49c9-b593-d223f7449a82
Initialize kses filters if _wp_unfiltered_html_comment nonce isn't set.

See #24752.


git-svn-id: http://develop.svn.wordpress.org/trunk@25137 602fd350-edb4-49c9-b593-d223f7449a82