FIX: composer problem on the categories page (#9508)

app/serializers/topic_list_item_serializer.rb

@@ -21,7 +21,7 @@ class TopicListItemSerializer < ListableTopicSerializer
   has_many :participants, serializer: TopicPosterSerializer, embed: :objects
 
   def posters
-    object.posters || []
+    object.posters || object.posters_summary || []
   end
 
   def op_like_count

spec/serializers/topic_list_item_serializer_spec.rb

@@ -83,5 +83,15 @@ describe TopicListItemSerializer do
 
       expect(json[:tags]).to eq([])
     end
+
+    it 'return posters' do
+      json = TopicListItemSerializer.new(topic,
+        scope: Guardian.new(user),
+        hidden_tag_names: [hidden_tag.name],
+        root: false
+      ).as_json
+
+      expect(json[:posters].length).to eq(1)
+    end
   end
 end