`??` message clarity and/or specification

```haxe
function main() {
Std.parseFloat("") ?? 1.0; // On static platforms, null can't be used as basic type Float
}
```

It's good that this fails, but the message isn't very clear because there's no visible `null` here. Can we detect this during typing and fail in a nicer way or will that anger the semantics crowd again?

Start by reproducing the Haxe snippet and reading how the compiler reports the `??` expression when `Std.parseFloat("")` is used with `Float`. Trace the typing and diagnostic path mentioned in the issue; done when the failure clearly explains the implicit nullability problem and the intended behavior is settled.

Written by the indexing model from the issue text.