Element.prototype.getAttributeNode has a non-nullable output type

[zaclummys]

TypeScript Version: 2.7.0-dev.20171229

Expected behavior:
Element.prototype.getAttributeNode should have the following output type: Attr | null

Actual behavior:
Element.prototype.getAttributeNode has a non-nullable output type: Attr

[mhegazy]

PRs welcomed. You can find more information about contributing lib.d.ts fixes at https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes.