if get_post_meta is empty echo a placeholder or shortcode

by

You can use empty, to check your variable.

Something like this

$prod_id = get_the_ID();
if (!empty(get_post_meta($prod_id,'_product_tech',true))) {
  echo'<a class="shutter" title="technische gegevens" href="'.get_post_meta($prod_id,'_product_tech',true).'"><img width="750" height="100%" src="'.get_post_meta($prod_id,'_product_tech',true).'"></a>';
} else {
   //Is empty
}