In section 15.5.4 (fn:doc), F+O states:

If the Available documents maps the string to an empty sequence, then the function returns an empty sequence. 

However, XPath section 2.1.2 says that available documents is a mapping from strings onto document nodes. It does not mention the possibility that it might map a string to an empty sequence. 

We need to decide which is right.

I don't personally see that any useful purpose is served by allowing a string to map to an empty sequence, and would delete this sentence from F+O.