Fix ToggleEvent source IDL

Since ToggleEvent's source property can return null, the IDL type needs to be Element? instead of Element. This matches CommandEvent's source, which is already Element?.

Fixes #11360.

Author: josepharhar

source

@@ -80564,7 +80564,7 @@ interface <dfn interface>ToggleEvent</dfn> : <span>Event</span> {
   constructor(DOMString type, optional <span>ToggleEventInit</span> eventInitDict = {});
   readonly attribute DOMString <span data-x="dom-ToggleEvent-oldState">oldState</span>;
   readonly attribute DOMString <span data-x="dom-ToggleEvent-newState">newState</span>;
-  readonly attribute Element <span data-x="dom-ToggleEvent-source">source</span>;
+  readonly attribute Element? <span data-x="dom-ToggleEvent-source">source</span>;
 };
 
 dictionary <dfn dictionary>ToggleEventInit</dfn> : <span>EventInit</span> {