Bug 10053: borrowers.branchcode cannot be null

Bug 10053: borrowers.branchcode cannot be null

The DB field borrowers.branchcode cannot be null (`branchcode`
varchar(10) NOT NULL default ''). However, an empty value can be selected
on creating/updating a patron.

Test plan:
- Add or edit a patron and choose the empty value for library
(branchcode).
- Save (pref BorrowerMandatoryField must not contain branchcode).
- Mysql raises an error.
- Apply the path.
- Check that you are not able to select no library.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>