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

git-svn-id: https://develop.svn.wordpress.org/trunk@28197 602fd350-edb4-49c9-b593-d223f7449a82