Ensure generator picks up ApplicationSerializer

ApplicationSerializer could exist, but not be loaded. So, we check
existence by looking at the filesystem instead of defined?.

Fixes https://github.com/rails-api/active_model_serializers/issues/1890

lib/generators/rails/serializer_generator.rb

@@ -25,7 +25,7 @@ module Rails
       def parent_class_name
         if options[:parent]
           options[:parent]
-        elsif defined?(::ApplicationSerializer)
+        elsif 'ApplicationSerializer'.safe_constantize
           'ApplicationSerializer'
         else
           'ActiveModel::Serializer'