Return early if start or stop are null

[koddsson]

Hey hey!

This patch should make sure that we don't throw when getVisibleRange returns a empty object:

https://github.com/clauderic/virtualized-list/blob/d519d57de207d73685a0230b9449e1d491d56353/src/VirtualList/SizeAndPositionManager.js#L121

Instead we just return early. Not sure if there is a better way to handle this case.

[MrPancakes39]

Why would start or end be null??