Insert image with empty ALT text without ()

When entering an image with the editor button, one is asked for the URL and an ALT text. As the ALT text is optional most people do not enter text here. This leads to an invalid syntax for Textile:

!https://qt.nokia.com/logo.png()!

The JavaScript should omit the () in that case in order to ensure a correct display:

!https://qt.nokia.com/logo.png!