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.md

@@ -1,7 +1,7 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 152.5s
+Wall time: 158.2s
 
 **Total:** 78/99 runnable passed (78.8%). Raw: pass=78 fail=15 skip=1 timeout=6 total=100.