LibWeb: Use NonnullGCPtr in EventTarget's event handler map

Null entries are not valid, so let's not allow them.

Userland/Libraries/LibWeb/DOM/EventTarget.h

@@ -73,7 +73,7 @@ class EventTarget : public Bindings::PlatformObject {
 
  // https://html.spec.whatwg.org/multipage/webappapis.html#event-handler-map
  // Spec Note: The order of the entries of event handler map could be arbitrary. It is not observable through any algorithms that operate on the map.
- HashMap<FlyString, JS::GCPtr<HTML::EventHandler>> event_handler_map;
+ HashMap<FlyString, JS::NonnullGCPtr<HTML::EventHandler>> event_handler_map;
  };
 
  Data& ensure_data();