Remove unnecessary loops in default-filters.php.

props Jayjdk.
fixes #25011.

Built from https://develop.svn.wordpress.org/trunk@25134


git-svn-id: http://core.svn.wordpress.org/trunk@25114 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Only show 'Preview' on the custom header screen if there is something to show.

props nickmomrik.
fixes #14531.

Built from https://develop.svn.wordpress.org/trunk@25135


git-svn-id: http://core.svn.wordpress.org/trunk@25115 1a063a9b-81f0-0310-95a4-ce76da25c4cd