Don't crash on AP request for tombstone

Because tombstone objects has no addressing the is_public?-predicate
would cause an error that propagated as a 500 error in the api

test/support/factory.ex

@@ -57,6 +57,19 @@ defmodule Pleroma.Factory do
     %Pleroma.Object{data: Map.merge(data, %{"to" => [user2.ap_id]})}
   end
 
+  def tombstone_factory do
+    data = %{
+      "type" => "Tombstone",
+      "id" => Pleroma.Web.ActivityPub.Utils.generate_object_id(),
+      "formerType" => "Note",
+      "deleted" => DateTime.utc_now() |> DateTime.to_iso8601()
+    }
+
+    %Pleroma.Object{
+      data: data
+    }
+  end
+
   def direct_note_activity_factory do
     dm = insert(:direct_note)