Bug 26232: Empty smart rule fine grace period

Bug 26232: Empty smart rule fine grace period

Staff can create smart rules with empty (undefined) fine grace period.
In Overdues.pm undefined firstremind means 0, do the same in Circulation.pm.

To test:
1. Apply these patches
2. Run:
   kshell
   prove t/db_dependent/Circulation.t

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>