telepathyClient: Don't ignore GAsyncReadyCallback

This is better for memory management, and we'll see any errors this
way, which we may eventually want to do something with.

We need to make this change because gjs recently started checking
(allow-none) on callbacks.

js/ui/telepathyClient.js

@@ -382,7 +382,7 @@ Source.prototype = {
         }
 
         let msg = Tp.ClientMessage.new_text(type, text);
-        this._channel.send_message_async(msg, 0, null);
+        this._channel.send_message_async(msg, 0, Lang.bind(this, function (src, result) { this._channel.send_message_finish(src, result);  }));
     },
 
     _presenceChanged: function (contact, presence, status, message) {