Update Validator.php

if the field is not required, and the value is empty, should return true.

![image](https://cloud.githubusercontent.com/assets/1205950/2575849/dcf06944-b95e-11e3-91b2-6e01396f2d7c.png)

when i find the model from database, the deadline attributes is null, then save it, it throw a validation error.