[FIX] sale_coupon: min quantity set to 0 will crash backend and shop

Before this commit, if minimum product quantity was set to 0 on a coupon
program, it would then throw:
  - On backend, when updating promotions
  - On frontend (shop), user won't access his cart (error 500)

Now, we prevent quantity from being lower than 1

Step to reproduce:
  - Create a coupon automatically applied, set quantity to 0, set reward to
    'Free Product' and set free product to iPad Mini
  - Go to /shop and add a iPad Mini to your cart, it will throw an error 500
    instead of redirecting to your cart
  - Go to backend, click on 'Update Promotions' on a quotation, it will show an
    error popup

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