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/securejoin/bobstate.rs

@@ -422,7 +422,7 @@ async fn send_handshake_message(
 ) -> Result<()> {
     let mut msg = Message {
         viewtype: Viewtype::Text,
-        text: Some(step.body_text(invite)),
+        text: step.body_text(invite),
         hidden: true,
         ..Default::default()
     };