[geary/mjog/rfc-reply-refs-critical] engine: Fix critical when replying to an email with zero message ids

[Michael Gratton <mjog src gnome org>]

commit 388414c2320b1c85047eeec7c330be78e0630ef6
Author: Michael Gratton <mike vee net>
Date:   Sun Apr 4 12:50:16 2021 +1000

    engine: Fix critical when replying to an email with zero message ids
    
    Don't ever return null from `Geary.RFC822.Utils.reply_references`
    return null, since its one call site in the client expects it to be
    non-null.

 src/engine/rfc822/rfc822-utils.vala | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/src/engine/rfc822/rfc822-utils.vala b/src/engine/rfc822/rfc822-utils.vala
index e0b0bee55..65ab43a7d 100644
--- a/src/engine/rfc822/rfc822-utils.vala
+++ b/src/engine/rfc822/rfc822-utils.vala
@@ -100,7 +100,7 @@ namespace Geary.RFC822.Utils {
     }
 
     /** Generate a References header value in reply to a message. */
-    public MessageIDList? reply_references(Email source) {
+    public MessageIDList reply_references(Email source) {
         var list = new Gee.LinkedList<MessageID>();
 
         // 1. Start with the source's References list
@@ -122,7 +122,7 @@ namespace Geary.RFC822.Utils {
             list.add(source.message_id);
         }
 
-        return (list.is_empty) ? null : new MessageIDList(list);
+        return new MessageIDList(list);
     }
 
     public string email_addresses_for_reply(MailboxAddresses? addresses,