What could I do, to not to have an empty area, when the javascript is disabled in the browser?

The Smart Slider needs javascript to work, so what you could do is to show an image for example instead of the slider with the help of noscript tags, which are only running, if the javascript loading is disabled. Go to the post or page, where you put the shortcode of your slider, switch to Text Editor, if you weren’t in that, and write this code under or over it with your slider’s id in the place of 1, and change the src of the image:

[smartslider2 slider="1"]
<noscript>
    <style>
    #nextend-smart-slider-1{
        display:none;
    }
    </style>
    <img src="http://www.nextendweb.com/demo/smartslider2/images/ken/k2.png">
</noscript>