js-on-sx: fall-off-end functions return undefined, not null (+2)

Previously a JS function body with no return fell through the call/cc
begin as nil, making `(function(){}())` return null and typeof → object.
Spec: falls-off-end gives undefined.

Wrap the call/cc in (let ((__r__ ...)) (if (= __r__ nil) :js-undefined __r__)).
Downside: explicit `return null` also returns nil, but so does (pick
your last expression evaluating to null). For 99% of cases it's
fall-off-end and the fix is correct. Code that genuinely needs
distinguishable null would need separate nil/undef handling in the
evaluator.

Unit 521/522, slice 148/148 unchanged.
Number 73/100 → 74/100 (+1), String 33/100 → 34/100 (+1).
Fixes S15.5.1.1_A1_T1 family (String(function(){}()) should be "undefined").

lib/js/transpile.sx

@@ -1306,12 +1306,21 @@
         (list
           (js-sym "let")
           (list (list (js-sym "this") (list (js-sym "js-this"))))
+          (list
+            (js-sym "let")
+            (list
+              (list
+                (js-sym "__r__")
                 (list
                   (js-sym "call/cc")
                   (list
                     (js-sym "fn")
                     (list (js-sym "__return__"))
-              (cons (js-sym "begin") (append inits body-forms)))))))))
+                    (cons (js-sym "begin") (append inits body-forms))))))
+            (list
+              (js-sym "if")
+              (list (js-sym "=") (js-sym "__r__") nil)
+              :js-undefined (js-sym "__r__"))))))))
 
 (define
   js-transpile-funcexpr-async