do not send notifications when listings have been filtered

lib/FredyRuntime.js

@@ -106,6 +106,9 @@ class FredyRuntime {
   }
 
   _notify(newListings) {
+    if (newListings.length === 0) {
+      throw new NoNewListingsWarning();
+    }
     const sendNotifications = notify.send(this._providerId, newListings, this._notificationConfig, this._jobKey);
     return Promise.all(sendNotifications).then(() => newListings);
   }