Use new null-safe generics in Pigeon

[gnprice]

The Pigeon feature request to handle types like List<Foo>, as opposed to only List<Foo?>, is complete:

• [pigeon] implement nullsafe generics flutter/flutter#97848 (comment)

We have a couple of types in our pigeon/notifications.dart that could benefit from that. One has a TODO comment; for the full list, look for List< and Map<.

[ArpanMondalGITHUB]

Can I work on it

[skylon07]

I also noticed that the docs should probably be updated too, since #5 is now obsolete.

[gnprice]

@skylon07 Good catch, yeah.

That'd be a good point to make as an issue in the Flutter upstream tracker: https://github.com/flutter/flutter/issues . This thread here is in the issue tracker of a particular app that uses Flutter, namely Zulip.

[skylon07]

Oh, haha, I clicked into this issue from the Flutter repo and didn't realize I had switched to Zulip. My apologies -- I'll make a ticket there instead.