Customize: Fix inability to delete nav menus by preventing preview filters from being added during customize_save admin ajax request.

Also prevent setting `nav_menu_locations[...]` values to `NaN` which gets sent as `null`.

Amends [38810].
See #30937.
Fixes #39103.


git-svn-id: https://develop.svn.wordpress.org/trunk@39558 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/js/customize-nav-menus.js

@@ -1177,7 +1177,11 @@
 
 			// @todo It would be better if this was added directly on the setting itself, as opposed to the control.
 			control.setting.validate = function( value ) {
-				return parseInt( value, 10 );
+				if ( '' === value ) {
+					return 0;
+				} else {
+					return parseInt( value, 10 );
+				}
 			};
 
 			// Edit menu button.