Accept toplevel action-producing expressions yielding 0 values.

A recent change to Racket must have changed the way `for` expands,
because now in conjunction with `local-expand`, we see *effectively* a
`(begin (values) (void))`. This isn't a problem usually, but in
`#lang syndicate`'s `module-begin` context, we split apart `begin`s
and examine their constituents, leading to examination of something
that will ultimately yield 0 values.

The change accepts either 0 or 1 values when collecting actions for
the module's main boot actor to execute. More than 1 value yielded by
such an expression is still considered an error. Currently, it gives
unhelpful error location information; a future refinement might be to
make the error reporting for this (rare) situation more helpful.

racket/syndicate/actor.rkt

@@ -1180,9 +1180,14 @@
      (with-store [(current-pending-actions '())
                   (current-pending-patch patch-empty)
                   (current-action-transformer values)]
-       (define result (thunk))
-       (flush-pending-patch!)
-       (cons result (current-pending-actions))))))
+       (call-with-values thunk
+                         (lambda results
+                           (flush-pending-patch!)
+                           (when (> (length results) 1)
+                             (error 'capture-actor-actions
+                                    "~a values supplied in top-level Syndicate action; more than one is unacceptable"
+                                    (length results)))
+                           (cons results (current-pending-actions))))))))
 
 (module+ for-module-begin
   (provide capture-actor-actions))