Do not check for file existence when serializing ActivityPub actor (#8386)

When serializing an ActivityPub actor, Mastodon checks for the existence
of the avatar/header files. This is not necessary, only check if avatar/header
is set.

fd8bf49d38/README.md (checking-a-file-exists)

app/serializers/activitypub/actor_serializer.rb

@@ -93,11 +93,11 @@ class ActivityPub::ActorSerializer < ActiveModel::Serializer
   end
 
   def avatar_exists?
-    object.avatar.exists?
+    object.avatar?
   end
 
   def header_exists?
-    object.header.exists?
+    object.header?
   end
 
   def manually_approves_followers