Rename 'get_search_form' action to 'pre_get_search_form' to prevent collision with the filter of the same name. Make sure the filtered result is not null to prevent search form from disappearing if an action function is attached to the old hook. fixes #19321.

git-svn-id: http://core.svn.wordpress.org/trunk@23800 1a063a9b-81f0-0310-95a4-ce76da25c4cd