Bug 28516: Prevent failures if borrower_attribute_types.mandatory=1

Bug 28516: Prevent failures if borrower_attribute_types.mandatory=1

TestBuilder will generate an integer for the
Koha::Patron::Attribute::Type object, but if 1 is picked some tests are
failing randomly
At least t/db_dependent/Koha/Patrons.t and t/db_dependent/Koha/Patrons/Import.t
The expection "Missing mandatory extended attribute" is raised when the
patron is stored.

Test plan:
The following script should return 0 when the patch is applied:

"""
use t::lib::TestBuilder;
my $builder = t::lib::TestBuilder->new;
my $x = $builder->build_object(
    {
        class => 'Koha::Patron::Attribute::Types',
    }
);
say $x->mandatory;
"""

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>