Bug 18357: Handle unlimited on-site checkouts

Bug 18357: Handle unlimited on-site checkouts

If on-site checkouts are set to unlimited (i.e. NULL/undef), they are
currently blocked.

Test plan:
1/ Set All/All rule with Unlimited/unlimited for normal/onsite checkouts
2/ Will be able to perform onsite checkout
3/ Edit rule to be 15/Unlimited normal/onsite
4/ Will be able to perform onsite checkout
=> Without this patch it was blocked
5/ Set rule to 15/15
6/ Onsite checkouts work again

Signed-off-by: David Bourgault <david.bourgault@inlibro.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>