messages: 'message-parts' accepts #f as the recipient.

* jupyter/messages.scm (message-parts): Allow #:recipients to be #f.

jupyter/messages.scm

@@ -122,7 +122,7 @@ with KEY, another string."
                         (recipient (and=> (message-parent-header message)
                                           header-sender)))
   "Return the list of parts (bytevectors) of MESSAGE for RECIPIENT, a ZeroMQ
-identity (a bytevector).
+identity (a bytevector) of #f if there's no specific recipient.
 
 This is a low-level procedure for internal use."
   (let* ((header    (header->string (message-header message)))
@@ -133,11 +133,16 @@ This is a low-level procedure for internal use."
          (content   (message-content message))
          (payload   (string-append header parent metadata content))
          (signature (get-signature key payload)))
-    (cons* recipient DELIM
-           (map string->utf8
-                (list signature
-                      header parent
-                      metadata content)))))
+    ;; Per
+    ;; <https://jupyter-client.readthedocs.io/en/latest/messaging.html#the-wire-protocol>,
+    ;; the identities part before DELIM "can be zero or more socket
+    ;; identities".  Here we accept zero or one.
+    `(,@(if recipient (list recipient) '())
+      ,DELIM
+      ,@(map string->utf8
+             (list signature
+                   header parent
+                   metadata content)))))
 
 (define (reply message type content)
   "Return a Jupyter message that is a reply to MESSAGE."