Add null safety and upgrade dependencies

[GiorgiBeriashvili]

Description

Resolves #7

[mattsolle]

Until this gets merged in you can point directly at the null safe version by including the following in your pubspec:

dio_flutter_transformer:
    git:
      url: https://github.com/GiorgiBeriashvili/dio_flutter_transformer
      ref: feature/null-safety

[sergey-s-moiseev]

It's need to merge!