Add fallback for missing subscribers (Shopify#3731)

Author: nathan-scheele

assets/pubsub.js

@@ -19,5 +19,7 @@ function publish(eventName, data) {
     const promises = subscribers[eventName]
       .map((callback) => callback(data))
     return Promise.all(promises);
+  } else {
+    return Promise.resolve()
   }
 }