re: https://immersive-web.github.io/webxr/#dom-xrframe-getviewerpose

The algortihms for these don't handle the case where the reference space is null.

It's unlikely the user will accidentially generate a null reference space because requestReferenceSpace(options) will reject if the created reference space is null. Although developers may pass in null as a mistake which should either be handled smoothly or cause an exception.