[Woo POS] Coupons: Handle state when there are no coupons in a store

[iamgabrielma]

Handle a case where the store has no coupons.

UI-wise we would have some sort of "You haven't created a coupon yet. Do you want to create one?" screen to show coupon creation CTA if no coupons are listed., but this bit will come later with the designs, specifics not applicable for now.

[dangermattic]

🚫 Please add a type label (e.g. type: enhancement) and a feature label (e.g. feature: stats) to this issue.