FIX: Don't publish polls on message bus when there are no polls (#15041)

`poll` plugin was publishing on `/polls/[topic_id]` every time a non-first post was created. I can't imagine this being needed. It regressed 3 years ago in https://github.com/discourse/discourse/pull/6359

plugins/poll/plugin.rb

@@ -160,11 +160,12 @@ after_initialize do
     guardian = Guardian.new(user)
     DiscoursePoll::Poll.schedule_jobs(post)
 
-    unless post.is_first_post?
+    next if post.is_first_post?
+    next if post.custom_fields[DiscoursePoll::HAS_POLLS].blank?
+
     polls = ActiveModel::ArraySerializer.new(post.polls, each_serializer: PollSerializer, root: false, scope: guardian).as_json
     post.publish_message!("/polls/#{post.topic_id}", post_id: post.id, polls: polls)
   end
-  end
 
   on(:merging_users) do |source_user, target_user|
     PollVote.where(user_id: source_user.id).update_all(user_id: target_user.id)

plugins/poll/spec/lib/poll_spec.rb

@@ -128,4 +128,41 @@ describe DiscoursePoll::Poll do
       )
     end
   end
+
+  describe "post_created" do
+    it "publishes on message bus if a there are polls" do
+      first_post = Fabricate(:post)
+      topic = first_post.topic
+      creator = PostCreator.new(user,
+        topic_id: topic.id,
+        raw: <<~RAW
+          [poll]
+          * 1
+          * 2
+          [/poll]
+        RAW
+      )
+
+      messages = MessageBus.track_publish("/polls/#{topic.id}") do
+        creator.create!
+      end
+
+      expect(messages.count).to eq(1)
+    end
+
+    it "does not publish on message bus when a post with no polls is created" do
+      first_post = Fabricate(:post)
+      topic = first_post.topic
+      creator = PostCreator.new(user,
+        topic_id: topic.id,
+        raw: "Just a post with definitely no polls"
+      )
+
+      messages = MessageBus.track_publish("/polls/#{topic.id}") do
+        creator.create!
+      end
+
+      expect(messages.count).to eq(0)
+    end
+  end
 end