FIX: Do not attempt to serialize Tag objects when tagging disabled (#18264)

When tagging is enabled, we were correctly serializing tags by their name. However, when tagging was disabled we were attempting to serialize an entire Tag object which raises an error since ee07f6da7d.

https://meta.discourse.org/t/232885

app/serializers/post_revision_serializer.rb

@@ -208,6 +208,7 @@ class PostRevisionSerializer < ApplicationSerializer
 
     # Retrieve any `tracked_topic_fields`
     PostRevisor.tracked_topic_fields.each_key do |field|
+      next if field == :tags # Special handling below
       if topic.respond_to?(field)
         latest_modifications[field.to_s] = [topic.public_send(field)]
       end

spec/requests/posts_controller_spec.rb

@@ -1750,6 +1750,23 @@ RSpec.describe PostsController do
         expect(response.status).to eq(200)
       end
     end
+
+    context "with a tagged topic" do
+      let(:tag) { Fabricate(:tag) }
+      it "works" do
+        SiteSetting.tagging_enabled = true
+
+        post_revision.post.topic.update(tags: [tag])
+
+        get "/posts/#{post_revision.post_id}/revisions/latest.json"
+        expect(response.status).to eq(200)
+
+        SiteSetting.tagging_enabled = false
+
+        get "/posts/#{post_revision.post_id}/revisions/latest.json"
+        expect(response.status).to eq(200)
+      end
+    end
   end
 
   describe '#revert' do