I don't believe that it's specified that "" is a reference to the
current document, it's just that that interpretation is a consequence
of other things that it does say.

This is because the resolution of schemaLocation="path/path/filename"
is taken as a relative URI from the base URI, which is (typically) the
current document (unless overridden specifically). So the degenerate
case of a relative URI is "", which when combined with base URI, is the
current document (unless base URI is set to something else, in which
case it's still base URI, but it's not the current document).

Your alternative also seems reasonable, if that's how your resolvers
work, but to do so you'd have to special case "". If you don't
special-case it, then "" == base URI (because appending nothing to base
URI has that effect). In your case, presumably you're treating it as an
absolute (null) URI, rather than as a relative URI?