media library not showing thumbnails after deleting full image

What is the need to remove the full size images? Are you dealing with storage space? If not, why give yourself the headache? Just let the full size images stay where they are. You don’t have to use them. You’ll probably find a situation where you want to use it anyways.

Also, I’ve never tried it, but you might be able to disable default image without deleting it by adding the following to your functions.php file:

update_option('full_size_w', '0');
update_option('full_size_h', '0');

You could also customize which image sizes display as an option in the editor as shown here.