Forgetting to return shouldn't wipe state

I get tripped up forgetting to add `return state` to the end of my action handlers, and it's a horrible and hard to figure out issue when suddenly part or all of your state is missing.

I propose simply checking if `typeof state === 'undefined'` and not changing the state if it is undefined. To wipe the state, one can `return null` or `return {}` or something.

Any opinions? Concerns?