[FIX] mail : fix post message callback error

When posting a message (here is the livechat use case),
the _postMessage function can call the callback function.
but this parameter is not mandatory.

If the caller of this function doesn't need a callback function
to be called after execution, the _postMessage function was crashing
as callback is not defined.

This commit fix this _postMessage function by checking that
callback is defined before calliing it.

Pre-required for Task ID: 2028059
Fix Task : 2037048
PR #34917

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/mail/static/src/js/thread_windows/abstract_thread_window.js

@@ -404,7 +404,7 @@ var AbstractThreadWindow = Widget.extend({
         this._thread.postMessage(messageData)
             .then(function () {
                 self._threadWidget.scrollToBottom();
-                callback();
+                callback && callback();
             });
     },
     /**