if a product only has images for its variants, display the first variant...

... image for the product instead of none

If a product only has images for its variants and no image for "all" variants, then a "No Image Available" image was shown. I've changed that so that the first variant image will be used instead. I believe this relates to what is mentioned in #2228

This is probably my first real PR, so I apologise if it's not correct and would love some guidance on how to improve it.