MDL-54804 templates: Prevent posting when no option is selected

When no value is set for an option element and a user selects it, a
POST request is made, which is incorrect. Usually that option is a
placeholder value, e.g., "Choose..." - the form expecting the end user
to choose an actual value.

lib/templates/url_select.mustache

@@ -74,6 +74,9 @@
     {{#js}}
         require(['jquery'], function($) {
             $('#{{id}}').change(function() {
+                if (!$(this).val()) {
+                    return false;
+                }
                 $('#{{formid}}').submit();
             });
         });