make set-union with 0 args return an empty set

collects/racket/set.rkt

@@ -111,6 +111,7 @@
 
 (define set-union
   (case-lambda
+   [() (set)]
    [(set) 
     (unless (set? set) (raise-type-error 'set-union "set" 0 set))
     set]

collects/tests/racket/set.rktl

@@ -17,6 +17,8 @@
 (test #t set-empty? (seteqv))
 (test #t set? (seteqv 1 2 3))
 (test #f set-empty? (seteqv 1 2 3))
+(test #t set? (set-union))
+(test #t set-empty? (set-union))
 
 (test #f set-eq? (set 1 2 3))
 (test #f set-eqv? (set 1 2 3))