Bug 11853: Allow to clear the date of birth at the OPAC

Bug 11853: Allow to clear the date of birth at the OPAC

A patron is not able to clear their date of birth.

When cleared tt is set to NULL in DB which means, for the patron's
modifications feature, that nothing has been changed.

Test plan:
0/ Do not apply the patch
1/ Edit your personal details at the OPAC
2/ Add a date of birth
3/ On the staff interface, approve the modification request
=> OK it is updated
4/ Edit your personal details and clear the date of birth
5/ On the staff interface
=> KO the table display is wrong, nothing is marked has changed
6/ Approve the modification requiest
=> KO it has not been cleared
7/ Apply the patch and repeat 1 to 6 and confirm that the behaviors are
now correct.

Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>