0 becomes null

[erjiang]

On pasukon.rocks, with the example grammar, type in 0 for the input. The result comes back as null, not 0. I've also written a test for this that I think demos the issue: erjiang@d9a6104

[erjiang]

This patch seems to work (tests pass, at least):

--- a/lib/parsers/result.js
+++ b/lib/parsers/result.js
@@ -4,7 +4,7 @@ module.exports = class Result {
   constructor ({ success, remaining, matched }) {
     this.success = success
     this.remaining = remaining
-    this.matched = matched || null
+    this.matched = matched == null ? null : matched
   }
 
   get failed () {