Customizer: Set a correct default for `nav_menu_locations` setting.

props westonruter.
fixes #33088.

git-svn-id: https://develop.svn.wordpress.org/trunk@33421 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-customize-nav-menus.php

@@ -496,6 +496,7 @@ final class WP_Customize_Nav_Menus {
 					'theme_supports'    => 'menus',
 					'type'              => 'theme_mod',
 					'transport'         => 'postMessage',
+					'default'           => 0,
 				) );
 			}