Bug 13502: Code introcduced in 1861 wrongly assumes a null userid is unique

Bug 13502: Code introcduced in 1861 wrongly assumes a null userid is unique

To test

1/ Create a borrower with '' as their userid, you may have to edit a
   row in the db to do this
2/ Run  perl t/db_dependent/Circulation/CheckIfIssuedToPatron.t
3/ Notice some tests fail and you see
   DBD::mysql::st execute failed: Duplicate entry '' for key 'userid'
   at /home/chrisc/git/catalyst-koha/C4/SQLHelper.pm line 184.
4/ Apply the patch
5/ Run the tests again, notice they now pass

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>