In slime-package-fu.el slime-determine-symbol-style mishandles the case when where are no exported symbols

[Orivej Desh]

This doesn't work as intended when symbols is nil: the first, not the
third case is matched.

(cond ((every (lambda (x)
                (string-match "^:" x))
              symbols)
       (lambda (n) (format ":%s" n)))
      ((every (lambda (x)
                (string-match "^#:" x))
              symbols)
       (lambda (n) (format "#:%s" n)))
      (t
       slime-export-symbol-representation-function))