Bug 29795: add empty value for branchcode on self-reg form

Bug 29795: add empty value for branchcode on self-reg form

1. Set branch code as a PatronSelfRegistrationBorrowerMandatoryField
2. Go to the self reg form and look at the Home library field, it defaults to the first choice alphabetically
3. Fill out the other required fields and the captcha. Don't change the branchcode value.
4. Submit the form, it submits.
5. If branchcode is required it should default to an empty value and force the user to make a choice.
6. Apply patch
7. Repeat 1-3
8. Now you cannot submit the form unless you choose a branchcode
9. Make sure it still works normally when branchcode is not in PatronSelfRegistrationBorrowerMandatoryField

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>