Bug 18887: Insert undef instead of '*'

Bug 18887: Insert undef instead of '*'

Koha::CirculationRules->get_effective_rule will become the method to
call to retrieve a specific rule, we should start using it when
possible.

Moreover undef could replace '*' to mean 'any', that way we will be able
to add FK on circulation_rules

TODO: Add more tests

Signed-off-by: Lisette Scheer <lisetteslatah@gmail.com>
Signed-off-by: Jesse Maseto <jesse@bywatersolution.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>