js-on-sx: Object(null) and Object(undefined) return new empty object

Per ES spec, Object(value) returns a new object when value is null
or undefined. Was returning the argument itself, breaking
Object(null).toString(). Added a cond clause to Object.__callable__
that detects nil/js-undefined and falls through to (dict).
built-ins/Object: 15/50 → 16/50. conformance.sh: 148/148.

lib/js/test262-scoreboard.json

@@ -56,6 +56,6 @@
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 152.5,
+  "elapsed_seconds": 158.2,
   "workers": 1
 }