[FIX] sale_coupon: Create a discount_line_product_id on program creation

When creating a program, we should always create a discount_line_product_id. Now on a product creation, if a product is not specified, the `vals` dictionary contains `discount_line_product_id: False` and the product is not created. It seems that the behavior on record creation has changed.

models/sale_coupon_program.py

@@ -67,7 +67,7 @@ class SaleCouponProgram(models.Model):
     @api.model
     def create(self, vals):
         program = super(SaleCouponProgram, self).create(vals)
-        if 'discount_line_product_id' not in vals:
+        if not vals.get('discount_line_product_id', False):
             discount_line_product_id = self.env['product.product'].create({
                 'name': program.reward_id.name_get()[0][1],
                 'type': 'service',