Bug 19750: Overdues without a fine rule add warnings to log

Bug 19750: Overdues without a fine rule add warnings to log

When running fines.pl, any overdue items that have no corresponding circulation rule will generate the following warning:

Use of uninitialized value $amount in numeric gt (>) at /usr/share/koha/bin/cronjobs/fines.pl line 133.

Test Plan:
1) Create a single circ rule
2) Backdate a checkout so it is overdue
3) Delete the circ rule
4) Run fines.pl, note the warning
5) Apply this patch
6) Run fines.pl, note the warning is gone

Signed-off-by: Dilan Johnpullé <dilan@calyx.net.au>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>