Add failing test for null hydration

[eps1lon]

This is only a failing test for inconsistent server vs client side behavior for null children. For undefined both server and client stringify them but for null only the server stringifies them. The client removes children if it encounters null.

Would like to fix those but I would need to know who's at fault here: client, server or user and where a good starting point for fixing this would be.

[Rich-Harris]

Thank you! This is a real edge case — it turns out that

text = document.createTextNode(null);

results in "null", but if you do

text.data = null

you get "". Go figure. The correct behaviour here would be for null to show up in both cases, so I've implemented that here: 3f33d35