Child shouldn't allow null or undefined

[thequailman]

https://github.com/MithrilJS/mithril.d.ts/blob/master/index.d.ts#L191

In my opinion, null | undefined should be removed from this type. It should be more strict, and users should have to use an empty array or placeholder vnode in order to avoid Mithril having to check if something is null or undefined.