code: updated to NullSafety

[ValeriusGC]

Simplest jump to null safety - a couple ? of changes in dart and one in yaml

[fewensa]

Are you willing to publish a library separately? If the author has not merged

[ValeriusGC]

Are you willing to publish a library separately? If the author has not merged

Done already in my fork.

It seems no sense to make duplicated library in pub. One can simply link to it via syntax like that:

  flutter_absolute_path:
    git:
      url: https://github.com/ValeriusGC/flutter_absolute_path.git