Fix never matching clause

`length/1` is only used with lists.

lib/pleroma/web/activity_pub/transmogrifier.ex

@@ -416,7 +416,7 @@ def handle_incoming(%{"type" => "Flag", "object" => objects, "actor" => actor} =
   def handle_incoming(%{"id" => nil}, _options), do: :error
   def handle_incoming(%{"id" => ""}, _options), do: :error
   # length of https:// = 8, should validate better, but good enough for now.
-  def handle_incoming(%{"id" => id}, _options) when not (is_binary(id) and length(id) > 8),
+  def handle_incoming(%{"id" => id}, _options) when is_binary(id) and byte_size(id) < 8,
     do: :error
 
   # TODO: validate those with a Ecto scheme