Bug 19908: Do not use .optional in password match validation function

Bug 19908: Do not use .optional in password match validation function

Test plan:
0) Aplly only first patch
1) You will be able to submit a from on member-password.pl even if the
passwords don't match
2) Apply this patch -> you can't be able to submit the form if the
paswords don't match, but you'll be able to submit the form when
password fields are blank

Signed-off-by: Roch D'Amour <roch.damour@inlibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>