fix(resolve schema reference): shortcut if definitions is undefined

[jpb06]

Hello. Firstof, thanks for maintaining this lib 🙇🏻

It appears this fix is causing an issue with our codebase. We have an endpoints that is is defined like this:

import type { FastifyInstance, FastifyRegisterOptions } from 'fastify';
import type { FastifyZodOpenApiTypeProvider } from 'fastify-zod-openapi';
import { z } from 'zod';

const myResponseSchema = z
  .object({
     // ...
  })
  .openapi({
    ref: 'MyResponse',
  });

fastifyInstance.withTypeProvider<FastifyZodOpenApiTypeProvider>().route({
    method: 'GET',
    url: '/',
    schema: {
      response: {
        [HTTPStatusCodeEnum.ok]: myResponseSchema
      },
    },

which resolves to a schema like this:

{
  "components": {
    "schemas": {
      "MyResponse": {
        "type": "object",
        "properties": {
           // ...
         }
      }
    }
  },
  "paths": {
    "/my-endpoint": {
      "get": {
        "responses": {
          "200": {
            "description": "Default Response",
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/MyResponse",
                }
              }
            }
          }
        }
      }
    }
  }
}

The issue is there is no definitions for this ref, which makes the whole thing crash. Wasn't the case before.
One simple fix is to check if resolvedReference.definitions is undefined before trying to access it.

I tried to write a test to reproduce the issue but I'm not an openapi expert so I can't seem to get anywhere:

test('should not crash if response has no definitions', async (t) => {
  const fastify = Fastify();

  await fastify.register(fastifySwagger, {
    openapi: {
      components: {
        schemas: {
          MyResponse: {
            type: 'object',
            properties: {},
          },
        },
      },
      paths: {
        '/response-with-ref': {
          get: {
            responses: {
              200: {
                description: 'Default Response',
                content: {
                  'application/json': {
                    schema: {
                      $ref: '#/components/schemas/MyResponse',
                    },
                  },
                },
              },
            },
          },
        },
      },
    },
  });
  fastify.register(async (instance) => {
    instance.get(
      '/response-with-ref',
      {
        schema: {
          response: {
            200: { $ref: '#/components/schemas/MyResponse' },
          },
        },
      },
      () => {}
    );
  });

  await fastify.ready();

  const openapiObject = fastify.swagger();

  t.equal(typeof openapiObject, 'object');

  await Swagger.validate(openapiObject);
});

This fails with:

 ✖should not crash if response has no definitions > Failed building the serialization schema for GET: /response-with-ref, due to errornode_modules/fastify/lib/route.js:
   Cannot find reference "#/components/schemas/MyResponse"                                                                            400:21

Any help would be appreciated. Thank you!

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct