Make NoPlaceholderTextPolicy history-aware

lib/pleroma/web/activity_pub/mrf/no_placeholder_text_policy.ex

@@ -6,14 +6,17 @@ defmodule Pleroma.Web.ActivityPub.MRF.NoPlaceholderTextPolicy do
   @moduledoc "Ensure no content placeholder is present (such as the dot from mastodon)"
   @behaviour Pleroma.Web.ActivityPub.MRF.Policy
 
+  @impl true
+  def history_awareness, do: :auto
+
   @impl true
   def filter(
         %{
-          "type" => "Create",
+          "type" => type,
           "object" => %{"content" => content, "attachment" => _} = _child_object
         } = object
       )
-      when content in [".", "<p>.</p>"] do
+      when type in ["Create", "Update"] and content in [".", "<p>.</p>"] do
     {:ok, put_in(object, ["object", "content"], "")}
   end