Fix error on fetching actor

I'm not sure what's actually causing it, but this prevents it from
clogging up the log.

lib/activitypub/federate.ts

@@ -72,7 +72,10 @@ async function cacheActor(actorObject: ActorObject) {
 	await getConnection().manager.save(actor);
 }
 
-async function fetchActor(url: string): Promise<ActorObject | null> {
+async function fetchActor(url?: string): Promise<ActorObject | null> {
+	if (!url) {
+		return Promise.resolve(null);
+	}
 	return new Promise((resolve, reject) => {
 		request({
 			url,