activitypub: don't fall back to OStatus fetching when MRF rejects an object

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -756,6 +756,9 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
            {:ok, activity} <- Transmogrifier.handle_incoming(params) do
         {:ok, Object.normalize(activity.data["object"])}
       else
+        {:reject, nil} ->
+          {:reject, nil}
+
         object = %Object{} ->
           {:ok, object}