feat: [MVUX] Improve nullable support

[dr1rrb]

closes #1605

Feature

Improve nullable support in feed factories and relevant operators

What is the current behavior?

Nullable annotation are not used to interpreted by the feeds. There is no recommendation to use non nullable types in feeds and users are often involuntary using using Some(null) while they should use (/ thinking they are using) None.

What is the new behavior?

• Most common factory methods (<Feed|State>.Async[Enumerable]) are now constrained with T : notnull
• Select and SelectAsync operators are also constrained with TResult : notnull
• When user returns a T with the T : struct constraint, we allowed him to return a T? which is interpreted as None
• In any time, when user provide a T that is ref type and null, it's interpreted as None (e.g. Feed<MyObject>.Async(async _ => null!) will now goes in None).
• SelectData and SelectDataAsync operators has been added to allow user to still use Some(null) if needed.

PR Checklist

• Tested code with current supported SDKs
• Docs have been added/updated which fit documentation template. (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Wasm UI Tests are not showing unexpected any differences. Validate PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Updated the Release Notes
• Associated with an issue (GitHub or internal)

Other information

COMPILATION BREAKING CHANGES

This PR introduces few compilation breaking changes. Here is a shot summary of the known breaking

• Feed.Async(async _ => default(MyObject)) is no longer allowed, to fix you need to specify the type of the value of the feed, i.e. use Feed**<MyObject?>**.Async(async _ => default(MyObject)). Same for Feed.AsyncEnumerable, State.Async and State.AsyncEnumerable
• myFeed.Select(_ => default(MyObject)) is no longer allowed, to fix you need to use the SelectData operator. Same for SelectAsync where you need use SelectDataAsync.

BEHAVIOR BREAKING CHANGES

This PR also introduces few behavior changes that could be breaking in few use case. Those are describe in the issue #1605, and which can be summarized "any time you return a null value, it's now automatically converted as None". This was already the case for some factories and operators and is expected to cause issue only in really rare use-cases. If you are encountering such an issue, use factories and operators overload that accepts Option<T> instead of T.

[dr1rrb]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[nickrandolph]

@dr1rrb how are we going to deal with the breaking changes. I assume we need to push to v4 of extensions, to reflect breaking changes?

[dr1rrb]

@dr1rrb how are we going to deal with the breaking changes. I assume we need to push to v4 of extensions, to reflect breaking changes?

I would agree, but usually (unfortunately) the "major" version number is more often a "marketing number" than a "semantic number"... especially since we have released v3 quite recently. We will have to discuss about that ...