Checking all coupons for potential empty string issues:

Coupon ID 3 (TUTI5OFF):
  usage_limit_per_customer: int(1)
  Type: integer
  Is null? NO
  Empty check: NO

Coupon ID 5 (ASEO10):
  usage_limit_per_customer: int(1)
  Type: integer
  Is null? NO
  Empty check: NO

Coupon ID 6 (TRONEX5):
  usage_limit_per_customer: int(1)
  Type: integer
  Is null? NO
  Empty check: NO

Coupon ID 7 (ETERNA8):
  usage_limit_per_customer: int(1)
  Type: integer
  Is null? NO
  Empty check: NO

Coupon ID 4 (SHAMPOO10):
  usage_limit_per_customer: int(1)
  Type: integer
  Is null? NO
  Empty check: NO