Fix error when trying to cache an actor without a public key

lib/activitypub/federate.ts

@@ -57,6 +57,8 @@ export async function getCachedActor(url: string): Promise<ActorObject | null> {
 }
 
 async function cacheActor(actorObject: ActorObject) {
+	if (!actorObject.publicKey) return;
+
 	function getIconUrl(icon: string | object): string {
 		return icon instanceof String ? icon : (icon as any).url;
 	}