DEV: Fix another MessageBus::InvalidMessageTarget error.

Follow-up to c2f2b57a9b.

app/models/topic_tracking_state.rb

@@ -438,11 +438,13 @@ SQL
     }
 
     channels.each do |channel, ids|
-      MessageBus.publish(
+      if ids.present?
-        channel,
+        MessageBus.publish(
-        message.as_json,
+          channel,
-        user_ids: ids
+          message.as_json,
-      )
+          user_ids: ids
+        )
+      end
     end
   end