fix: Make pre-messages w/o text want MDNs (#8004)

This fixes failing `test_pre_msg_mdn_before_sending_full`. Maybe it's even good that MDNs will be
sent for pre-messages only having placeholder text with the viewtype and size, at least this way we
notify the contact that we've noticed the message. Anyway, with adding message previews the problem
will be solved for the corresponding viewtypes.

Author: iequidoo

src/mimeparser.rs

@@ -957,14 +957,16 @@ impl MimeMessage {
         self.parse_attachments();
 
         // See if an MDN is requested from the other side
+        let mut wants_mdn = false;
         if self.decryption_error.is_none()
-            && !self.parts.is_empty()
+            && (!self.parts.is_empty() || matches!(&self.pre_message, PreMessageMode::Pre { .. }))
             && let Some(ref dn_to) = self.chat_disposition_notification_to
         {
             // Check that the message is not outgoing.
             let from = &self.from.addr;
             if !context.is_self_addr(from).await? {
                 if from.to_lowercase() == dn_to.addr.to_lowercase() {
+                    wants_mdn = true;
                     if let Some(part) = self.parts.last_mut() {
                         part.param.set_int(Param::WantsMdn, 1);
                     }
@@ -986,7 +988,9 @@ impl MimeMessage {
                 typ: Viewtype::Text,
                 ..Default::default()
             };
-
+            if wants_mdn {
+                part.param.set_int(Param::WantsMdn, 1);
+            }
             if let Some(ref subject) = self.get_subject()
                 && !self.has_chat_version()
                 && self.webxdc_status_update.is_none()