Bug 28345: Patron attributes no longer have option to select empty class

Bug 28345: Patron attributes no longer have option to select empty class

This patch adds the "empty" parameter to the PROCESS directive which
builds the "Class" dropdown. This allows the user to choose a blank
option when creating or editing a patron attribute.

To test, apply the patch and go to Administration -> Patron attribute
types -> Add patron attribute type.

In the entry form the "Class" dropdown should have a blank option. When
editing an existing attribute with a class defined, the correct class
should still be preselected.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>