activitypub: help ecto build a better query for thread mute filtering

using an indexed value in thread_mute table helps ecto build a better query.

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -796,7 +796,7 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
       )
 
     unless opts["skip_preload"] do
-      from([thread_mute: tm] in query, where: is_nil(tm))
+      from([thread_mute: tm] in query, where: is_nil(tm.user_id))
     else
       query
     end