如果函数中没有提供参数，我该如何测试它是否留空？

<?php    
/* Copyright Date
--------------------------*/
function copyright_date($creation_year) {
    $current_year = date('Y');

    if ($creation_year == $current_year || $creation_year == '') {
        echo $current_year;
    } 
    else {
        echo $creation_year . '-' . $current_year;
    }           
}
?>
<?php    
/* Copyright Date
-------------------