Wordpress media library missing entries with NULL value when sorted after custom column

i have included 2 custom columns in the metadata of media entries called 'photographer' and 'copyright'. I have included both columns in the admin view of the media library and made them sortable. But when i sort by one of them the results exclude entries with a NULL value in that column

adding custom fields to admin panel media view

//Custom columns in Media Admin view

add_filter('manage_media_columns', 'media_additional_columns', 1);
function media_additional_columns($defaults){
    $defaults['photographer'] = __('Photographer');
    $defaults['copyright'] = __('Copyright');
    return $defaults;
}

//Fill Custom Media Admin Columns with content

add_action('manage_media_custom_column', 'media_custom_columns_attachment_id', 1, 2);
function media_custom_columns_attachment_id($column_name, $post_id){
   switch ( $column_name ) {
    case 'photographer':
      echo get_post_meta( $post_id, 'photographer', true );
      break;

    case 'copyright':
      echo get_post_meta( $post_id, 'copyright', true );
      break;
}
}

make custom columns in admin panel media view sortable

//make custom columns sortable

//add custom columns to array sortable columns
add_filter('manage_upload_sortable_columns', 'as_add_custom_column_sortable');
function as_add_custom_column_sortable($columns) {
    $columns['photographer'] = 'photographer';
    $columns['copyright'] = 'copyright';
    return $columns;
  }

//alter the post query in case stuff gets sorted by custom columns
  add_action( 'pre_get_posts', 'as_custom_column_orderby' );
  function as_custom_column_orderby( $query ) {
    if( ! is_admin() || ! $query->is_main_query() ) {
      return;
    }
    switch ($query->get( 'orderby')) {
      case 'photographer':
        $query->set( 'orderby', 'meta_value' );
        $query->set( 'meta_key', 'photographer' );
        break;
      case 'copyright':
        $query->set( 'orderby', 'meta_value' );
        $query->set( 'meta_key', 'copyright' );
        break;   
    }
  }

I want to include the entries with NULL value in the sorted table, after the entries with non-NULL values.

I have found this post that answers the same question for the Posts and for Users. I thought the answer applicable for Posts would work also for media since media are basically posts of the type attachment but it doesnt, my media library turns up completely empty.

I am just starting with php, help would be much appreciated!

alrighty i circumvented the problem by putting the code into a plugin file instead of functions.php directly and adding code that updates the values of the custom columns for all images that already exist in the media library once the plugin is activated

////update all images with default values for custom fields "Photographer" and "Copyright"
// Run the loop when the plugin is activated
register_activation_hook(__FILE__, 'as_update_my_metadata');
function as_update_my_metadata(){
    $args = array(
        'post_type' => 'attachment', // Only get the posts
        'post_status' => 'publish' OR 'private', // Only the posts that are published
        'posts_per_page'   => -1 // Get every post
    );
    $posts = get_posts($args);
    foreach ( $posts as $post ) {
        // Run a loop and update every meta data
        update_post_meta( $post->ID, 'photographer', 'unknown' );
        update_post_meta( $post->ID, 'copyright', 'not defined' );
    }
}