parser-combinators: fix 'satisfy' parser for empty strings

contrib/parser-combinators/parser-combinators.factor

@@ -35,10 +35,14 @@ M: satisfy-parser (parse) ( input parser -- list )
   #! A parser that succeeds if the predicate,
   #! when passed the first character in the input, returns
   #! true.
+  over empty? [
+    2drop nil
+  ] [
     satisfy-parser-quot >r unclip-slice dup r> call [
       swap <parse-result> 1list
     ] [
       2drop nil
+    ] if 
   ] if ;
 
 TUPLE: epsilon-parser ;