`refToNode` doesn't need to take a Nullable value

[natefaubion]

I don't see why refToNode needs to take a Nullable. The corresponding purescript-react API returns a Maybe Ref, so it has to be converted to Nullable, and then back to Maybe just to use this API. Since it's just a coercion, why not just export an unsafe coercion? I think it's probably better to do it safely through purescript-dom's Foreign based readers, though.