It seems that a search for nothing returns no result, and adding qualifiers like category and user still return nothing (this is related to the issues described at Search sometimes tokenizes unique user names and some times does not).
One of my forum users wants to see all their posts in a particular category, so they search for:
but that fails because there is no match.
A lousy workaround is to search for:
username:myname category:mycategory mycategory
Since mycategory always appears (or appears to appear) in the search matches, this finds everything.
But it seems to me that the search system should accept a search of only options.