Remove items with quantity 0 on cart page

Readd line removed in e07f4569f2dd63f411d81b6cf380140a73ac1de1 and add
spec to prove why it needs to be there for now

2-0-stable needs the fix as well. Currently the cart page display items with quantity 0 as they're not removed

ps. see the discussion in https://github.com/spree/spree/pull/3339#discussion_r5078990 for more details