Forbid empty string input to string->preserve

implementations/racket/preserves/main.rkt

@@ -496,6 +496,8 @@
 (define (string->preserve s)
   (define p (open-input-string s))
   (define v (read-preserve p))
+  (when (eof-object? v)
+    (parse-error* p "Unexpected end of input"))
   (skip-whitespace* p)
   (when (not (eof-object? (peek-char p)))
     (parse-error* p "Unexpected text following preserve"))