Price should not be calculated when store has no pricing rule set

[alexsegura]

When a store has no pricing rule set, the price is calculated anyway.

Moreover, when there are no pricing rules sets defined, we should show a better error message inviting to create one, instead of a blank dropdown.

[Paul-Eraman-CoopCycle]

For a new cooperative admin user who has a fresh new instance, when they are making their first store. When they make the store, we want a message of some kind that immediately says "great! now make a price rule over here [link]"