fix: don't leak Group-ID in Message-ID

Chat assignment based on In-Reply-To and References works good enough
even if the message cannot be decrypted.

src/context.rs

@@ -1359,7 +1359,7 @@ mod tests {
              \n\
              hello\n",
             contact.get_addr(),
-            create_outgoing_rfc724_mid(None, contact.get_addr())
+            create_outgoing_rfc724_mid(contact.get_addr())
         );
         println!("{msg}");
         receive_imf(t, msg.as_bytes(), false).await.unwrap();