Menu Customizer: properly account for potentially empty or invalid changed settings in menu previewing. Fixes a fatal error with Menu Customizer and WordPress 4.1, caused by only posting dirty settings to the Customizer preview in core.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@1039678 b8457f37-d9ea-0310-8a92-e5e31aec5664