switch global null to come from Nullable module and not Null

[zth]

This changes the globally available null value to originate from Nullable rather than Null. Makes sense imo given that global null and undefined now originate from the same module.

It also makes working with domRefs in React more natural, allowing you to write null directly instead of Nullable.null:

@react.component
let make = () => {
  let divRef = React.useRef(null)
  <div ref={divRef->ReactDOM.Ref.domRef}> {"Hello"->React.string} </div>
}

(side note - would be cool to get completion for ReactDOM.Ref.* in the scenario below. Might be able to work that into the editor tooling)

Thoughts? cc @cknitt @glennsl

[cknitt]

I like it! 🙂

[glennsl]

Ditto! :)

[cristianoc]

What about typical bindings, is the idea that one most often encounters null | undefined | T, rather than null | T?
Asking as the few binding I looked at to add genType support to Core were using null | T.
Otherwise one has o bind with Null.t and will have most often to reach for Null.null.

[glennsl]

Good question. I seems to recall the decision was made long ago to prefer Nullable.t over Null.t and Undefined.t, just for simplicity. I don't think React refs have ever been set to undefined, so I believe that is the reason why it's typed as it is.

I wonder if it would be practical to make Null.t a subtype of Nullable.t...