Repro for mercurius-validation nullable types issue

index.js

'use strict'

const Fastify = require('fastify')
const mercurius = require('mercurius')
const mercuriusValidation = require('mercurius-validation')
const assert = require('assert')

const app = Fastify()

const schema = `
  type Query {
    test(in: String): String
  }
`

const resolvers = {
  Query: {
    test: async (_, args) => {
      assert(args.in === null)
      return 'it works'
    }
  }
}

app.register(mercurius, {
  schema,
  resolvers
})
app.register(mercuriusValidation)

app.listen(3210)

package.json

{
  "name": "test-graphql",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "dependencies": {
    "fastify": "^3.27.0",
    "graphql": "^15.8.0",
    "mercurius": "^8.12.0",
    "mercurius-validation": "^1.0.0"
  }
}