Bug 21046: Return the correct borrowernumber when there is empty cardnumber(s)

Bug 21046: Return the correct borrowernumber when there is empty cardnumber(s)

Test plan:
  - Use a patron with an empty cardnumber to authenticate with ILSDI
    AuthenticatePatron
    (cgi-bin/koha/ilsdi.pl?service=AuthenticatePatron&username=userid&password=pass),
  - make sure you have other patron(s) with empty cardnumber and there
    borrowernumber is
    smaller than the one you authenticate with,
  - you should get a wrong borrowernumber,
  - apply this patch,
  - test again, you should get the right one

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>