Bug 30788: Fix warning in Overdues.pm when fine is empty in circ rules

Bug 30788: Fix warning in Overdues.pm when fine is empty in circ rules

Argument "" isn't numeric in multiplication (*) at /usr/share/koha/lib/C4/Overdues.pm

To test:
* Create a issuing rule where fine is empty, but a fine interval is set.
  Fine amount: empty
  To do this, remove the 0 displayed in the input field before saving.
  Fine charging interval: 5 or any other numeric value
  When to charge: Start of interval
  Fine grace period: 0
* Check out an item with the due date yester (use specify due date)
* Run misc/cronjobs/fines.pl -v
* You should see above warn in the output
* Apply patch
* Rerun fines.pl, there should be no warn.
* Run:
  t/db_dependent/Circulation/CalcFine.t
  t/db_dependent/Fines.t
  t/db_dependent/Circulation.t
  t/db_dependent/Overdues.t

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>