Check Li Arrays More Thoroughly

When you're checking if a multidimensional array is set you need to check for all of the previous dimensions are set first. This will prevent certain types of errors like this which I saw with my MailPoet extension.

> Notice: Uninitialized string offset: 0 in /ninja-forms/wp-content/plugins/ninja-forms/includes/admin/edit-field/li.php on line 91