Fix empty sync messages being sent

This happened when giving `send_message` a `SyncMessage` to one self.
In that case, the message would not be sent and instead
`create_multi_device_sent_transcript_content` would create a new sync
message, which is empty as the `SyncMessage` is neither a `DataMessage`
nor an `EditMessage`.

This PR changes the condition to ignore sync messages.

src/sender.rs

@@ -358,6 +358,8 @@ where
     ) -> SendMessageResult {
         let content_body = message.into();
         let message_to_self = recipient == &self.local_aci;
+        let sync_message =
+            matches!(content_body, ContentBody::SynchronizeMessage(..));
         let is_multi_device = self.is_multi_device().await;
 
         use crate::proto::data_message::Flags;
@@ -370,7 +372,7 @@ where
         };
 
         // only send a sync message when sending to self and skip the rest of the process
-        if message_to_self && is_multi_device {
+        if message_to_self && is_multi_device && !sync_message {
             debug!("sending note to self");
             let sync_message = self
                 .create_multi_device_sent_transcript_content(