Bug 24259: Handle non existing default circ rule in Charges::Fees

Bug 24259: Handle non existing default circ rule in Charges::Fees

Can't call method "lengthunit" on an undefined value at
/usr/share/koha/lib/Koha/Charges/Fees.pm line 101.

Test plan:
1 - Remove all/all rule form circulation rules
2 - Define a rental fee per day for an itemtype
3 - Checkout an item of this type ot patron
4 - Should get note of no circ rule defined
5 - Confirm the checkout
=> item is checked out

Signed-off-by: hc <hc@interleaf.ie>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>