correct behavior of blank curlies

brag/rules/parser.rkt

@@ -168,7 +168,9 @@
                         [(list all min range? max) (let* ([min (if min (string->number min) 0)]
                                                           [max (cond
                                                                  [(and range? max) (string->number max)]
-                                                                 [(and (not range?) (not max)) min] ; {3} -> {3,3}
+                                                                 [(and (not range?) (not max)) (if (zero? min)
+                                                                                                   #f ; {} -> {0,}
+                                                                                                   min))] ; {3} -> {3,3}
 [else #f])])
 (cons min max))]))]
 [else (raise-argument-error 'grammar-parse "unknown repetition operator" $2)]))