fix(notifications): skip the filter check for objects without content

Regex.match? raised on objects without content (for example posts with
only attachments), so no notifications were created for them when the
recipient had an irreversible filter.

lib/pleroma/notification.ex

@@ -821,6 +821,9 @@ defmodule Pleroma.Notification do
       object.data["actor"] == user.ap_id ->
         false
 
+      not is_binary(object.data["content"]) ->
+        false
+
       not is_nil(regex = Pleroma.Filter.compose_regex(user, :re)) ->
         Regex.match?(regex, object.data["content"])