ActivityPub: Change ordering to `nulls last` in favorites query

This makes it use our existing index and speeds up the query.

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -1318,7 +1318,7 @@ def fetch_favourites(user, params \\ %{}, pagination \\ :keyset) do
     |> Activity.with_joined_object()
     |> Object.with_joined_activity()
     |> select([_like, object, activity], %{activity | object: object})
-    |> order_by([like, _, _], desc: like.id)
+    |> order_by([like, _, _], desc_nulls_last: like.id)
     |> Pagination.fetch_paginated(
       Map.merge(params, %{"skip_order" => true}),
       pagination,