Still wrong nullable attributes

[FvS-MR]

As in https://github.com/zzzprojects/html-agility-pack/issues/587 there are still missing attributes,

E.g.

public HtmlNode FirstChild
{
	get { return !HasChildNodes ? null : _childnodes[0]; }
}

public HtmlNode LastChild
{
	get { return !HasChildNodes ? null : _childnodes[_childnodes.Count - 1]; }
}