Is there a action hook for the “Empty Trash” button?

I don’t think there exist one, but you could create your own, wpse_empty_trash, with something like this:

/**
 * Add a custom hook 'wpse_empty_trash'
 */
add_action( 'load-edit.php', function()
{
    add_action( 'before_delete_post', function ( $post_id )
    {
        if (
            'trash' === get_post_status( $post_id ) 
            && filter_input( INPUT_GET, 'delete_all' )
            && 1 === did_action( 'before_delete_post ' )
        )   
            do_action( 'wpse_empty_trash' );
    } );
} );

Then you can use it with your code. Example:

add_action( 'wpse_empty_trash', 'myFunction' );
function myFunction() {
    // My code
}

Hopefully you can adjust this to your needs.