fix(nag): fetch missing event on manual dismiss call

When the dismiss function is called manually via behavior .nag('dismiss') rather than being called inside an event (like when clicking on the close icon) a JS console error occured.

src/definitions/modules/nag.js

@@ -146,8 +146,10 @@
                         module.debug('Dismissing nag', settings.storageMethod, settings.key, settings.value, settings.expires);
                         module.storage.set(settings.key, settings.value);
                     }
-                    event.stopImmediatePropagation();
-                    event.preventDefault();
+                    if (event) {
+                        event.stopImmediatePropagation();
+                        event.preventDefault();
+                    }
                 },
 
                 should: {