Handle omitted exit code correctly

[matteovhaxt]

Hey there ✌🏻,
I stumbled upon this issue in my application, because supabase auth omits the exitCode. This resulted in the number being incorrectly displayed when being parsed by the PhoneController.

To reproduce this give the PhoneInput in the example app a controller with a default value like this (notice the omitted +):

final number = '4917612345678';
...
PhoneInput(
   controller: PhoneController(PhoneNumber.parse(number)),
   ...
   ),

This will result in the PhoneInput showing the correct countryCode but not removing the countryCode from the nsn. Effectively showing the user this faulty output:

Instead of the correct number:

Thanks for building the package though, handling phone numbers is always a great pain which this package helped me with, a lot! 👍🏻