onConfirmSignup accepts null for success whereas onLogin, onSignUp accepts empty string for success

[tushxr16]

Describe the bug
To maintain consistency with the existing functions which accept empty string as a measure of success, the onConfirmSignup should also follow the same behavior.

To Reproduce
Steps to reproduce the behavior:

1. Go to FlutterLogin( onConfirmSignup: (String code, LoginData data) => _onConfirmSignup(context, code, data) )
2. onConfirmSignup code should be like this to maintain consistency with other functions
   Future<String> _onConfirmSignup( BuildContext context, String code, LoginData data) async { // logic return ''; }
3. Instead we have to in following way
   Future<String?> _onConfirmSignup( BuildContext context, String code, LoginData data) async { // logic return null; }