fix(active_job): remove nil from global serializers set during app re-initialization

Author: solnic

sentry-rails/spec/dummy/test_rails_app/config/applications/rails-latest.rb

@@ -43,6 +43,15 @@ def cleanup!
         ActiveSupport.filter_parameters.clear
       end
 
+      # Re-initialising the app triggers the active_job.custom_serializers
+      # initializer which calls add_serializers(app.config.active_job.custom_serializers).
+      # Under some Rails/Ruby combinations custom_serializers resolves to nil instead
+      # of the railtie default of [], inserting nil into the global serializers Set.
+      # Remove it here so it does not outlive this app instance.
+      if defined?(::ActiveJob::Serializers)
+        ::ActiveJob::Serializers._additional_serializers.delete(nil)
+      end
+
       super
     end
   end