the_excerpt filter with an empty excerpt

If you are talking about excerpt set manually in the excerpt field of post then you can use following code in the functions.php file of your child theme or plugin to display default excerpt if no excerpt it set.

function display_default_excerpt( $excerpt ) {

    if ( has_excerpt() ) {
        return $excerpt;
    } else {
        return __( 'There is no excerpt so displaying default excerpt.' );
    }
}
add_filter( 'the_excerpt', 'display_default_excerpt' );