Update Validator.php

if the field is not required, and the value is empty, should return true.

···
public $validates = array(
		'name' => array('notEmpty', 'message' => 'name must not be empty'),
		'project_id' => array(
			array('notEmpty', 'message' => 'project_id must not be empty'),
			array('decimal', 'message' => 'project_id must be decimal'),
		),
		'deadline' => array('date', 'required' => false, 'format' => 'ymd', 'message' => 'deadline must be date fomat'),
);

···

when i find the model from database, the deadline attributes is null, then save it, it throw a validation error.