bug 2287: handle case where autoMemberNum is off

bug 2287: handle case where autoMemberNum is off

When importing a patron file where new records
do not have a card number *and* autoMemberNum is
off, make sure that bororwers.cardnumber is set to NULL
for those new patrons rather than '' - otherwise, only
one patron with an empty barcode can be created, as there is
a uniqueness constraint on borrowers.cardnumber

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>
Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>