Null callback shortcut?

gistfile1.txt

Stumbled upon this, the code at the bottom compiles, but there is no output, 
when I was thinking it might output something like >> list 123, 456

Was looking for a shorthand to 

final cb = callback;
if (cb != null) {
  cb(['123','456']);
}

I know I can use callback!(['123','456']) inside conditional, but I would have expected callback?(['123','456']) 
would also compile, but it doesn't.

Any chance there is a way to make it work as I guessed?


import 'package:test/test.dart';

typedef RegisteredClientListCallback = void Function(List<String> clientIds);
main() {
  RegisteredClientListCallback? callback = (clientIds) => print('list $clientIds');
  test('description', () {
    callback ??
        (cb) {
          cb(['123', '456']);
        };
  });
}