Bug 34818: (bug 21083 follow-up) Fix batch patron modification tool

Bug 34818: (bug 21083 follow-up) Fix batch patron modification tool

It is not possible to perform batch patron modification without selecting
a patron attribute

Test plan:
Test the batch patron modification tool, with or without extended
attributes.

The error was
Can't call method "category_code" on an undefined value at /kohadevbox/koha/tools/modborrowers.pl line 407

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>