TrayIcon.displayMessage does not display ballon when message value is set to NULL .

Step to Reproduce
---------------------
1. Run the Attached Program.after copying the image also to the same directory.
2. Click the Checkbox 'Add a system tray icon' to add the tray icon to the system tray.
   You can see tray icon on the system tray.
3. Type in some value for the message and caption and click the button 'Show the Message'.
   You can see the ballon near the trayIcon.
4. click on "Show NULL Caption" button. You can see the ballon near the trayIcon with
   only Message in the ballon.
5. Click on "Show NULL Message" button. If you dn't see ballon near the trayIcon , then
   bug is reproduced.

I tested this with Mustang b51 to b73.