I can't point you to any specific documentation, but yes,
JSF, like HTML, is quite averse to NULL data. And I can tell you from painful experience that NULL values in databases can be only slightly less injurious to one's sanity than attempting to use dates as database keys.
I prefer Calendar date controls like many extension tagsets offer anyway. If I had a date item and really truly wanted it to be nullable, I'd generally use a façade property on the backing bean and use that to populate the nullable persistent property that it corresponded to. I would then handle the null values as part of the general store/fetch code for that property.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.