Force users to choose a nav menu in the custom nav menu widget, for a better customizer UX

Before, they had to make a dummy change to get it to render. Now they
are made to choose a nav menu from the dropdown, which feels more
natural.

fixes #27878 for trunk. props westonruter
Built from https://develop.svn.wordpress.org/trunk@28197


git-svn-id: http://core.svn.wordpress.org/trunk@28027 1a063a9b-81f0-0310-95a4-ce76da25c4cd