Null safety support

[nonameden]

Migrated package to support Null safety, following guide: https://dart.dev/null-safety/migration-guide

Linked issue: #59

[IchordeDionysos]

@LinusU does this look good to you and are you willing to merge the PR? :)

[jonasfranz]

Looks good to me. I've tested this branch with the oauth2_client library. Found no bugs so far.

[nonameden]

1 month past, still not merged, is anything blocking it? @LinusU

[ThexXTURBOXx]

It seems that the author doesn't have interest in maintaining this project anymore as he is working on various other things.

@LinusU I would happily help maintaining this project if you need help, i.e. adding me as collaborator. Many users need this library for their application to be updated appropriately.
If you want, just reach out to me and we can dicuss this further.

[IchordeDionysos]

@LinusU I know you are probably very busy 😌
Because of that, I think we all would appreciate it even more if you could take a look at this PR and release a null-safe version to pub.dev. 😍

[okrad]

I second that... The lack of null safety support of this library is what prevents oauth2_client to be considered fully null safe.
@LinusU, I would really appreciate if you could merge this PR (and #42!). I would gladly help in any way.

[albertpurnama]

Great work @nonameden and thanks to @LinusU for maintaining this awesome project.

Let's merge this soon!

[EthanShoeDev]

Thanks for migrating this! I would also love it if @LinusU could merge the changes. Thanks for the awesome package!

[chrislaurie-io]

I also need the null safety. Is it maybe an idea for @okrad to fork this? Especially as it is closely linked to his oauth client package and then we can have null safety in our apps?

[LinusU]

Sorry for the delay everyone!

Fixed in 🚢 0.3.0 / 2021-04-06

[albertpurnama]

Thanks @LinusU !

[okrad]

Thank you @LinusU!!!