我无法理解这里的\\uu构造的功能。我知道在任何类中，只要类被实例化，\\uu构造就会运行。然而，我好奇的是，当$marks在\\uu construct函数中定义为null时，为什么它要在\\uu construct的if语句中检查其值？它不会总是返回false吗？

class Marks_Manage
{
    public $mark_id;
    public $exam_id;
    public $class_id;
    public $subject_id;
    public $marks;
    public $attendance;
    public $student_id;
    public $marks_comment;
    public $created_date;
    //current_time( \'mysql\' );

    public function __construct( $marks = null ) 
    {
        if($marks)
        {
            global $wpdb;
            $table_name = $wpdb->prefix . "marks";
            $mark_data = $wpdb->get_row( "SELECT * FROM $table_name WHERE mark_id=".$marks);
            $this->mark_id = $mark_data->mark_id;
            $this->exam_id = $mark_data->exam_id;
            $this->class_id = $mark_data->class_id;
            $this->subject_id = $mark_data->subject_id;
            $this->marks = $mark_data->marks;
            $this->attendance = $mark_data->attendance;
            $this->student_id = $mark_data->student_id;
            $this->marks_comment = $mark_data->marks_comment;

        }
    }