admin_title() defaults to 'Untitled', if $pagetitle is empty.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5246 2fea6a4d-a838-0410-917b-93a53c48e9d2
Use 'untitled' as title when editing an article without a title.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5247 2fea6a4d-a838-0410-917b-93a53c48e9d2