Fix the Node interface (microsoft#11113)

Author: falsandtru

Diff for: lib/lib.dom.d.ts

@@ -8747,18 +8747,18 @@ interface Node extends EventTarget {
     readonly attributes: NamedNodeMap;
     readonly baseURI: string | null;
     readonly childNodes: NodeList;
-    readonly firstChild: Node;
-    readonly lastChild: Node;
+    readonly firstChild: Node | null;
+    readonly lastChild: Node | null;
     readonly localName: string | null;
     readonly namespaceURI: string | null;
-    readonly nextSibling: Node;
+    readonly nextSibling: Node | null;
     readonly nodeName: string;
     readonly nodeType: number;
     nodeValue: string | null;
     readonly ownerDocument: Document;
-    readonly parentElement: HTMLElement;
-    readonly parentNode: Node;
-    readonly previousSibling: Node;
+    readonly parentElement: HTMLElement | null;
+    readonly parentNode: Node | null;
+    readonly previousSibling: Node | null;
     textContent: string | null;
     appendChild(newChild: Node): Node;
     cloneNode(deep?: boolean): Node;