php json_decode删除具有null值的属性

I have a Json string and I am decoding it using php's json_decode.

The string

            "address": {
                "address": null,
                "postalCode": null,
                "phoneNumber": "",
                "city": null
            }

When I decode the string I get

            ["address"]=>
                  array(1) {
                  ["phoneNumber"]=>
                       string(0) ""

It essentially strips the attributes with null as a value i.e address, city. Can I prevent this from happening.

COMPLETE JSON

            {"cost": null,
            "receiptNumber": null,
            "receiptType": null,
            "labNo": 596726,
            "parentLabNo": 0,
            "investigation": "BS for mps",
            "patient": {
                "id": 168967,
                "fullName": "UVOGIN",
                "dateOfBirth": "1972-04-04 00:00:00",
                "gender": "Male"
            },
            "address": {
                "address": null,
                "postalCode": null,
                "phoneNumber": "",
                "city": null
            }
        }