Null safety version

[mzdm]

Hello,
this package is one of the last that blocks my app from migrating to null safety.
I would appreciate if you would publish null safe version of this package. If you don't have time I have raised a PR for this #7.

Thank you.

@kontinuity

At worst I could temporarily publish null safe version of this package till you are active again.

[mzdm]

As this was the last blocking package, I have temporary published a null safety version (till the maintainer comes back):
https://pub.dev/packages/clean_settings_nnbd