Bug 24114: (follow-up) Resolve warning on non-numeric subtraction

Bug 24114: (follow-up) Resolve warning on non-numeric subtraction

Argument "" isn't numeric in subtraction (-) at /usr/share/koha/Koha/Patrons.pm line 290.

Coming from an empty or undefined FailedLoginAttempts.

Test plan:
Verify that Koha/Patrons.t still passes.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Bouzid Fergani <bouzid.fergani@inlibro.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Joy Nelson <joy@bywatersolutions.com>