Half filled time is considered null

[beauchette]

I have this form in which I tried using , so, the field value is null when there's no time entered, and the time itself when it's entered. Great. My problem that this field is optional, but if a user just types 10:-- and then blurs it, the value is still null, while the user think he just input 10:00, and I can't detect the error because I allow it to be null.