LibJS: Fix string representation for value of type undefined

Author: https://github.com/elisee
Commit: https://github.com/SerenityOS/serenity/commit/cbe0053a97e
Pull-request: https://github.com/SerenityOS/serenity/pull/1376

Libraries/LibJS/Value.cpp

@@ -39,7 +39,7 @@ String Value::to_string() const
         return "null";
 
     if (is_undefined())
-        return "null";
+        return "undefined";
 
     if (is_number()) {
         // FIXME: This needs improvement.