Prevent crashing when a component is null

[joshwnj]

I'm seeing this error in the console when a component returns null from its render function: TypeError: Object.defineProperty called on non-object

I've added some failing specs and modified core/snapshot.js to make them pass.

PS. thanks for this library!

[MicheleBertoli]

This is awesome, thank you very much @joshwnj.
Good catch, and thanks for adding some tests as well.