Fix: greater_than usage (#2909)

## Context

Credit Note Item model allows amount_cents to be 0; Because of
roundings, applied_coupon.amount_cents can also be 0

app/models/applied_coupon.rb

@@ -25,7 +25,7 @@ class AppliedCoupon < ApplicationRecord
 
   monetize :amount_cents, disable_validation: true, allow_nil: true
 
-  validates :amount_cents, numericality: {greater_than: 0}, allow_nil: true
+  validates :amount_cents, numericality: {greater_than_or_equal_to: 0}, allow_nil: true
   validates :amount_currency, inclusion: {in: currency_list}, allow_nil: true
 
   def mark_as_terminated!(timestamp = Time.zone.now)