FIX: Error when trying to bump a topic with no category (#21207)

When revising a post, if the topic that post belonged to did not have a category attached it would error with 

> NoMethodError (undefined method `read_restricted' for nil:NilClass)

app/models/topic_tracking_state.rb

@@ -579,6 +579,8 @@ class TopicTrackingState
   def self.secure_category_group_ids(topic)
     category = topic.category
 
+    return [Group::AUTO_GROUPS[:admins]] if category.nil?
+
     if category.read_restricted
       ids = [Group::AUTO_GROUPS[:admins]]
       ids.push(*category.secure_group_ids)