[FEATURE] Generate null for nullable custom scalars

[Stranger6667]

Is your feature request related to a problem? Please describe.
At the moment, if a custom scalar is nullable, it is skipped from arguments. But null is a valid value, which is not generated. Example query:

{
  countries(filter: {code: {eq: null}}) {
    languages {
      name
      native
    }
  }
}

Describe the solution you'd like
Generate NullValueNode for such cases