Fix autocomplete product without image

[shochdoerfer]

I had an issue with the autocomplete results not rendering due to this error:

An exception has been thrown during the rendering of a template ('Parameter "path" for route "liip_imagine_filter" 
must match ".+" ("" given) to generate a corresponding URL.') in
"@GallySyliusPlugin/shop/shared/components/header/search/autocomplete/results.html.twig"

I added a bit of logic in the template to also deal with products that don't have images set.

[shochdoerfer]

Was not sure which branch to merge it in. Since the 2.2 branch has more commits than master, I decided to use this branch as the target.