Allow null for XPathResult.singleNodeValue (#662)

From the specification [1]:

> The value of this single node result, which may be null.

[1] https://www.w3.org/TR/DOM-Level-3-XPath/xpath.html#XPathResult

Author: badeball

baselines/dom.generated.d.ts

@@ -17423,7 +17423,7 @@ interface XPathResult {
     readonly invalidIteratorState: boolean;
     readonly numberValue: number;
     readonly resultType: number;
-    readonly singleNodeValue: Node;
+    readonly singleNodeValue: Node | null;
     readonly snapshotLength: number;
     readonly stringValue: string;
     iterateNext(): Node;

inputfiles/browser.webidl.preprocessed.json

@@ -11322,6 +11322,7 @@
                             "specs": "dom-level-3-xpath",
                             "exposed": "Window",
                             "name": "singleNodeValue",
+                            "nullable": 1,
                             "type": "Node",
                             "type-original": "Node",
                             "read-only": 1