fix(containment): return nil for actor shapes without an id

Containment.get_actor raised on remote shapes it did not know, such as
an attributedTo object without an id. This crashed object fetches and
poll refetches. Return nil, so containment rejects the object.

lib/pleroma/object/containment.ex

@@ -44,6 +44,9 @@ defmodule Pleroma.Object.Containment do
     id
   end
 
+  # remote data can carry any shape here; callers treat nil as "no actor"
+  def get_actor(_), do: nil
+
   def get_object(%{"object" => id}) when is_binary(id) do
     id
   end