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/stock_str.rs

@@ -1313,7 +1313,7 @@ impl Context {
         chat::add_device_msg(self, Some("core-welcome-image"), Some(&mut msg)).await?;
 
         let mut msg = Message::new(Viewtype::Text);
-        msg.text = Some(welcome_message(self).await);
+        msg.text = welcome_message(self).await;
         chat::add_device_msg(self, Some("core-welcome"), Some(&mut msg)).await?;
         Ok(())
     }