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/authres.rs

@@ -705,7 +705,7 @@ Authentication-Results: dkim=";
         let received = tcm
             .try_send_recv(&alice, &bob2, "My credit card number is 1234")
             .await;
-        assert!(!received.text.as_ref().unwrap().contains("1234"));
+        assert!(!received.text.contains("1234"));
         assert!(received.error.is_some());
 
         tcm.section("Turns out bob2 wasn't an attacker at all, Bob just has a new phone and DKIM just stopped working.");
@@ -786,7 +786,7 @@ Authentication-Results: dkim=";
             .insert_str(0, "List-Post: <mailto:deltachat-community.example.net>\n");
         let rcvd = alice.recv_msg(&sent).await;
         assert!(!rcvd.get_showpadlock());
-        assert_eq!(&rcvd.text.unwrap(), "hellooo in the mailinglist again");
+        assert_eq!(&rcvd.text, "hellooo in the mailinglist again");
 
         Ok(())
     }