gistfile1.php

<?php
add_filter( 'cac/column/value', 'myplugin_cac_column_featured_image_value', 10, 4 ); 

function myplugin_cac_column_featured_image_value( $value, $postid, $column, $post_type ) {

	if ( $column->properties->type == 'column-featured_image' ) {
		if ( ! $value ) {
			$value = '<em>' . __( 'No featured image', 'myplugin' ) . '</em>';
		}
	}

	return $value;
}
?>