when ever you have a featured image in a query there is a chance you wont always have one. It would be nice if on the query loop settings there was a default image to use when no featured image is attached to the post. I often make custom blocks verses using the query loop because of this, or else I have to enforce a featured image on every post.
you can do this globaly with a hook. it will then apply to query block too. there is also a nice plugin that does this thing. Default featured image – WordPress plugin | WordPress.org