Datepicker cannot be cleared when onChange callback is defined

[chuyler]

If you define a parser and onChange handler similar to demo example, the user is unable to set the date input back to empty by clicking and removing the content.

After investigation, it is because the parser passes back an empty string '' instead of Date and date-picker.jsx is trying to call date.getTimezoneOffset().

[chuyler]

Figured out how to pass invalid date to avoid error.