Empty areas should render so fast (thanks to adaptivity) that they aren't really a "problem". There's no temporal adaptivity at the moment, which is "if all of a frame is mostly the same apart from one small area, focus on that small area and just copy and paste the previous frame for the rest", not even sure that is possible given how those areas might all interplay with, say, reflections, caustics etc. (but that's what feature suggestions are for, starting discussion!)