prebirth: Correct token value parsing

Empty string was not matching, which was causing its value to be '""', which
expanded (in JS) into the compiled string '""""'.  Oops.

* build-aux/bootstrap/prebirth.js (Parser#_token): Distinguish empty values
    from non-matches.

build-aux/bootstrap/prebirth.js

@@ -268,7 +268,7 @@ class Parser
         const token = {
             type:   type,
             lexeme: lexeme,
-            value:  value || lexeme,
+            value:  ( value === undefined ) ? lexeme : value,
             pos:    pos
         };