Checking all coupons for potential empty string issues:

Coupon ID 121 (PRUEBA5):
  usage_limit_per_customer: NULL
  Type: NULL
  Is null? YES
  Empty check: YES

Coupon ID 117 (PRUEBA1):
  usage_limit_per_customer: NULL
  Type: NULL
  Is null? YES
  Empty check: YES

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

Coupon ID 125 (PRUEBA9):
  usage_limit_per_customer: NULL
  Type: NULL
  Is null? YES
  Empty check: YES

Coupon ID 122 (PRUEBA6):
  usage_limit_per_customer: NULL
  Type: NULL
  Is null? YES
  Empty check: YES

Coupon ID 118 (PRUEBA2):
  usage_limit_per_customer: NULL
  Type: NULL
  Is null? YES
  Empty check: YES

Coupon ID 126 (PRUEBA10):
  usage_limit_per_customer: NULL
  Type: NULL
  Is null? YES
  Empty check: YES

Coupon ID 123 (PRUEBA7):
  usage_limit_per_customer: NULL
  Type: NULL
  Is null? YES
  Empty check: YES

Coupon ID 119 (PRUEBA3):
  usage_limit_per_customer: NULL
  Type: NULL
  Is null? YES
  Empty check: YES

Coupon ID 127 (PRUEBA12):
  usage_limit_per_customer: NULL
  Type: NULL
  Is null? YES
  Empty check: YES

Coupon ID 120 (PRUEBA4):
  usage_limit_per_customer: NULL
  Type: NULL
  Is null? YES
  Empty check: YES

Coupon ID 124 (PRUEBA8):
  usage_limit_per_customer: NULL
  Type: NULL
  Is null? YES
  Empty check: YES

Coupon ID 128 (PRUEBA11):
  usage_limit_per_customer: NULL
  Type: NULL
  Is null? YES
  Empty check: YES