[Enhancement] Support for nullability annotations

[bdovaz]

Microsoft and the community is continuously working on making the migration where possible.

Currently in WPF it's also proposed: dotnet/wpf#7563

Since you are working on a new version, it is a good time to support it.

If you need help, I have no problem creating a PR for this.

[dansiegel]

You'll notice that this is starting to work it's way in one API at a time. Unfortunately it's not going to be the quickest or easiest thing as broadly enabling nullability will break too many things and require too many changes. We are happy to take small focused PRs towards this end though.

https://github.com/PrismLibrary/Prism/blob/91beae021cfeefef4bc68bbcc89da29500d727b3/src/Prism.Core/Commands/AsyncDelegateCommand.cs#L8C1-L8C17

[bdovaz]

First attempt in #2959

[dansiegel]

Please hold off on any more PRs for this until after we do the Prism 9 preview. But absolutely appreciate the help to start getting these updated.