[BUGFIX] Support empty/NULL for DateOfBirth Day, Month and Year

Return by default NULL for getDateOfBirthDay(),
getDateOfBirthMonth() and getDateOfBirthYear() so input fields
show empty when no DateOfBirth is set.

Also only set new dateOfBirth by setDateOfBirthDay(),
setDateOfBirthMonth() and setDateOfBirthYear() if all
values aren't NULL.

If you want only the DateOfBirthYear to be set you have to
provide 2 hidden fields for DateOfBirthDay and DateOfBirthMonth
with the value 1. Else DateOfBirthYear will not be set.

Change-Id: Ia90f005ff40b031a15e8098e3307f0adf98bb53d
Reviewed-on: http://review.typo3.org/33012
Reviewed-by: Sebastian Fischer <typo3@evoweb.de>
Tested-by: Sebastian Fischer <typo3@evoweb.de>