fix: assign MDNs to the trash chat early

Otherwise we will try to create an ad-hoc group
and failing because there are only two contacts
and then unblock a 1:1 chat just to assign
the message to trash in the end.

src/receive_imf.rs

@@ -560,6 +560,11 @@ async fn add_parts(
             markseen_on_imap_table(context, rfc724_mid).await.ok();
         }
 
+        if chat_id.is_none() && is_mdn {
+            chat_id = Some(DC_CHAT_ID_TRASH);
+            info!(context, "Message is an MDN (TRASH).",);
+        }
+
         if chat_id.is_none() {
             // try to assign to a chat based on In-Reply-To/References: