Featured image fallback link to permalink

I have updated your code so that the fallback image has post permalink. I hope this helps:

if ( has_post_thumbnail() ) {
   echo '<a href="' . get_permalink($post->ID) . '" >';
   the_post_thumbnail();
   echo '</a>';
} else {
        echo '<a href="' . get_permalink($post->ID) . '" ><img src="'. get_stylesheet_directory_uri() . '/img/fallback-featured-image.jpg" /></a>';
}