FIX: flaky specs after topic view custom filters (#13019)

* FIX: flaky specs after topic view custom filters

When ensuring TopicView class variables return to the original state it should use empty Hash instead of empty Array. That

https://github.com/discourse/discourse/blob/master/lib/topic_view.rb#L60

* FIX: convert to string for topic view custom filter

spec/requests/topics_controller_spec.rb

@@ -2456,7 +2456,7 @@ RSpec.describe TopicsController do
 
           expect(body["post_stream"]["posts"].map { |p| p["id"] }).to eq([post3.id])
         ensure
-          TopicView.instance_variable_set(:@custom_filters, [])
+          TopicView.instance_variable_set(:@custom_filters, {})
         end
       end
     end