Avoid null values completely

I am using mackaroo API to generate JSON objects. Although I have specified “percentBlank”:0 for all the attributes I am requesting via the API, some return values this contain null. Example:
[
{“percentBlank”:0,“name”:“number”,“type”:“Regular Expression”,“value”:"\d{5}"},
{“percentBlank”:0,“name”:“zip”,“type”:“Postal Code”},
{“percentBlank”:0,“name”:“state”,“type”:“State”},
{“percentBlank”:0,“name”:“country”,“type”:“Country”},
{“percentBlank”:0,“name”:“city”,“type”:“City”}
]

generated

{
“number”: “21673”,
“country”: “China”,
“city”: “Hexi”
}

Can you please explain?