refactor: add codec for Optional

TICKET: WP-6196

Author: danielzhao122

modules/express/openapi-generator.rc.js

@@ -1,13 +1,13 @@
 /**
  * OpenAPI Generator Configuration
- * No custom configuration needed for Express module so far.
- * NOTE: This file is not being passed into the openapi-generator
+ * Custom codec handlers for OpenAPI generation
  */
 
 module.exports = (E) => {
   return {
     '.': {
       JsonFromStringifiedJson: () => E.right({ type: 'string' }),
+      Optional: (args) => E.right({ ...args[0], nullable: true }),
     },
     'io-ts-types': {
       Json: () => E.right({ type: 'object', properties: {}, required: [] }),