redex-match violates its documented contract

This patch fixes the problem that the hole matcher may return an empty
list rather than a #f that gets sent to the caller of redex-match. I
re-ran the Redex tests and found no violations. However, I am not
confident that there isn't a more correct place to put this '() -> #f
replacement.

I will be immediately forwarding this push email to Robby to have him
check it.

collects/redex/private/matcher.rkt

@@ -1324,7 +1324,10 @@ See match-a-pattern.rkt for more details
             (let loop ([mtches mtches]
                        [acc null])
               (cond
-                [(null? mtches) acc]
+                [(null? mtches) 
+                 (if (null? acc)
+                   #f
+                   acc)]
                 [else 
                  (let* ([mtch (car mtches)]
                         [bindings (mtch-bindings mtch)]

collects/redex/tests/tl-test.rkt

@@ -424,6 +424,10 @@
     (test (and (redex-match L -b 100) #t) #t)
     (test (redex-match L -b 3) #f))
 
+  (let ()
+    (test (redex-match empty-language number 'a) #f)
+    (test (redex-match empty-language (in-hole hole number) 'a) #f))
+
   (parameterize ([current-namespace (make-base-namespace)])
     (eval '(require redex/reduction-semantics redex/pict))
     (eval '(define-language L