data-flickity-lazyload-srcset without data-flickity-lazyload-src

If I prefer to set the srcset only by lazyLoad, the src attribute is set to null. Old browsers without srcset support will then not display the image. I must always set the data-flickity-lazyload-src attribute.

**Test case:** https://codepen.io/kajman-cz/pen/bGpwyEw

Looks like you just have to adjust
```
// set src & serset
this.img.src = src;
```
to
```
// set src & srcset
if ( src ) {
this.img.src = src;
}
```

Start with the lazy-load image code containing the shown src and srcset assignment, then reproduce the behavior using the linked CodePen test case. Done means lazy-loading with only data-flickity-lazyload-srcset no longer assigns a null src, while the existing src behavior remains unchanged.

Written by the indexing model from the issue text.