DEV: Fix another `MessageBus::InvalidMessageTarget` error.

DEV: Fix another MessageBus::InvalidMessageTarget error.

Follow-up to c2f2b57a9bbd9fdef74d1bde7a71de1abbcc08c5.

diff --git a/app/models/topic_tracking_state.rb b/app/models/topic_tracking_state.rb
index 6380c80..9f8b5a6 100644
--- a/app/models/topic_tracking_state.rb
+++ b/app/models/topic_tracking_state.rb
@@ -438,11 +438,13 @@ SQL
     }
 
     channels.each do |channel, ids|
-      MessageBus.publish(
-        channel,
-        message.as_json,
-        user_ids: ids
-      )
+      if ids.present?
+        MessageBus.publish(
+          channel,
+          message.as_json,
+          user_ids: ids
+        )
+      end
     end
   end
 

GitHub sha: 49f53bdc