Bug 31381: Handle null attributes in list

Bug 31381: Handle null attributes in list

This patch allows null values to be returned in patron attributes
to prevent a crash when searching patrons

To test:
1 - Add a NULL attribute to a borrower, in sample data, Edna Acosta
    sudo koha-mysql kohadev
    INSERT INTO borrower_attributes (borrowernumber,code,attribute) VALUES (5,'SHOW_BCODE',NULL);
2 - Browse to 'Home->Patrons'
    http://localhost:8081/cgi-bin/koha/members/members-home.pl
3 - Click 'Browse by last name: A'
4 - Datatables error:
    Expected string - got null
5 - Apply patch
6 - Restart all
7 - Browse by last name: A
8 - Success!

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>