NodeListOf#item should return TNode | null

Regarding this issue: https://github.com/microsoft/TypeScript-DOM-lib-generator/pull/1268

Real return type: https://developer.mozilla.org/en-US/docs/Web/API/NodeList/item

It would be much better for item() in NodeListOf to match behaviour with at() for arrays. The small difference is that it can return null instead of undefined.