It's really odd that objectStores and unique indexes can store just a single item with type null.  At TPAC, we discussed why we allow null and decided that we should remove null from the list of valid key types and instead come up with a way to say that a particular keyPath is optional for indexes.  (This is a bigger discussion that needs to happen, and I believe Jonas is going to start it on public-webapps in the near future about this.)