Set positional constructor param as optional

[RodolfoSilva]

Right now the RouteGuard interface use guardedRoute param as nullable, but the constructor requires the positional param.

The constructor guardedRoute param should also be optional.

Now you will be able to implements the interface without the constructor:

class MyGuard extends RouteGuard {
  @override
  Future<bool> canActivate(String path, ModularRoute router) async {
    // TODO: implement canActivate
    throw UnimplementedError();
  }
}