Make GitHub.auth non-nullable

[CaseyHillers]

For the most part, this is already non-nullable.

See flutter/flutter#129134 - This should help with debugging why our server is getting a null auth state.

[kevmoo]

We'll want a changelog entry here, right?

[CaseyHillers]

We'll want a changelog entry here, right?

Changelog entries are generated by the GitHub action

[CaseyHillers]

@robbecker-wf can you help release this? \cc @ricardoamador

[robrbecker]

Released in version 9.15.0 https://github.com/SpinlockLabs/github.dart/releases/tag/9.15.0

[Hixie]

I don't mind making this non-nullable, but can we restore making it mutable? GitHub credentials regularly need to be refreshed and having to recreate a whole new GitHub object each time is more annoying that just giving the existing object a new authentication token.

[CaseyHillers]

I don't mind making this non-nullable, but can we restore making it mutable?

Out of curiosity, do you have sample code I could take a look at? This seems like a fair request, but I was under the impression most callers will recreate the entire client instead of re-minting the auth.

[Hixie]

https://github.com/Hixie/cocoon/blob/9b487e12dab09c2eb396ce9aa9b5ee4a76991756/triage_bot/lib/engine.dart#L1147