Discount codes patch: 500 internal server error

```
 File "/data/app/app/api/schema/discount_codes.py", line 153, in validate_quantity
2019-08-10T16:48:03.322594701Z     if data['tickets_number'] < data['max_quantity']:
2019-08-10T16:48:03.322599976Z TypeError: '<' not supported between instances of 'int' and 'NoneType'
```