<Null<T>> to <T> variance

The following works on all targets except HL:

```haxe
class Main {
static function main() {
function foo(a:Array) trace(a);
foo(([1, 2, 3] : Array>));
}
}
```

```
Uncaught exception: Can't cast #hl.types.ArrayObj to #hl.types.ArrayBytes_Int
Called from $Main.main(Main.hx:4)
Called from fun$337(?:1)
Error: Command failed with error 1
```

Haxe version: 37f6b2a9b
HashLink version: HaxeFoundation/hashlink@1b09bbb