lose Null<T> in DynamicAccessKeyValueIterator so the object inlining is not prevented (closes #10118)

std/haxe/iterators/DynamicAccessKeyValueIterator.hx

@@ -48,6 +48,6 @@ class DynamicAccessKeyValueIterator<T> {
 	**/
 	public inline function next():{key:String, value:T} {
 		var key = keys[index++];
-		return {value: access[key], key: key};
+		return {value: (access[key] : T), key: key};
 	}
 }