api!: make Message.text non-optional

Message.set_text() and Message.get_text() are modified accordingly
to accept String and return String.

Messages which previously contained None text
are now represented as messages with empty text.
Use Message.set_text("".to_string())
instead of Message.set_text(None).

src/imap.rs

@@ -412,7 +412,7 @@ impl Imap {
                     drop(lock);
 
                     let mut msg = Message::new(Viewtype::Text);
-                    msg.text = Some(message.clone());
+                    msg.text = message.clone();
                     if let Err(e) =
                         chat::add_device_msg_with_importance(context, None, Some(&mut msg), true)
                             .await