If a nav menu has no items, wait until after the wp_nav_menu_items filter before deciding whether to print nothing.

see [21868] for original commit. see #21576.
see #24035 for trunk.



git-svn-id: http://core.svn.wordpress.org/trunk@24634 1a063a9b-81f0-0310-95a4-ce76da25c4cd