Make sure concrete shopping cart Daos behave consistently when fetching by id finds nothing.

[BasilBibi]

Concrete shopping cart Daos should return None when fetching by id.

e.g. get_customer_by_id should return None if the given customer id is not found in the underlying data set.

[ZakBibi]

All shoppingcart Daos behave consistently when fetching by id finds nothing