DEV: Ensure a broken tag_group relation doesn't raise an error (#16529)

A category_required_tag_group should always have an associated tag_group. However, this is only enforced at the application layer, so it's technically possible for the database to include a category_required_tag_group without a matching tag_group.

Previously that situation would cause the whole site to go offline. With this change, it will cause some unexpected behavior, but the site serializer will not raise an error.

app/serializers/category_required_tag_group_serializer.rb

@@ -4,6 +4,6 @@ class CategoryRequiredTagGroupSerializer < ApplicationSerializer
   attributes :name, :min_count
 
   def name
-    object.tag_group.name
+    object.tag_group&.name
   end
 end