Add opportunity to directly assign undefined for optional properties

[Denllay]

Right now, we cannot assign the undefined property value, even if it is optional. So it forces to write additional boilerplate to return empty object instead of undefined property. For example:

const getProperty = () => {
  if (condition) {
    return { property };
  }

  return {};
}

...

request: {
...getProperty()
...
}

// instead of

request: {
property: condition ? property : undefined
}

[igorkamyshev]

I've added some tests (add5a6c) for this use-case, and it's working correct 🤔

Can you elaborate a bit or provide reproduce of broken types?