[Reactive] Deprecate State.Empty

[dr1rrb]

What would you like to be added:

Deprecate State.Empty

Why is this needed:

This is miss-leading of its behavior when used in VM for 2-way bindings on de-normalized entities, most of the time a State.Value(this, () => new T()) is what user are looking for (cf. https://github.com/unoplatform/uno.extensions/discussions/1163)

For which Platform:

• [x] iOS
• [x] Android
• [x] WebAssembly
• [x] WebAssembly renders for Xamarin.Forms
• [x] Windows
• [ ] Build tasks

Anything else we need to know?

We should also consider the possibility to add a State<T>.SOMETHING(this) when T has a parameter-less constructor. Quid of the right name of this method:

• Default would be misleading with default(T)
• Empty would conflict with previous method and introduce a silent breaking change

[nickrandolph]

Without State.Empty how would you represent a state that hasn't been set (equivalent of a null value)