fix sf4 deprecated: cannotBeEmpty() is not allowed with arrayNode

Widget/PersonListWidgetFactory.php

@@ -44,7 +44,7 @@ class PersonListWidgetFactory extends AbstractWidgetFactory
                 ->info("Add some custom field to the view. Add the slug of some custom field"
                         . " if you want to override the view and show their value in the list")
                 ->example(array("custom-field-slug-1", "custom-field-slug-2"))
-                ->cannotBeEmpty()
+                ->requiresAtLeastOneElement()
                 ->end();
     }