Fixes centering of lightbox when no image present

I ran into this while using lightbox for UI, instead of an image.

Because I haven't set resizeToFit to false, the lightbox jumped to the center of the window, since no image was included and therefore the size was being taken as 0s.

You might also consider replacing
    $img = that.$element.find('.lightbox-content').find('img:first')
by looking up the topmost child, regardless its tag.