Ability to omit fields in WebService reply when they are optional

I'm working with the integrated web services server and have run into a problem. My service has several optional elements, but I can't seem to find a way to indicate that it's optional so they are returned as blank elements.

It is a restful interface if that matters.

I have a service that has several optional parameters, that are only passed back in certain conditions. But there doesn't seem to be a way to designate a tag as optional, so I get this JSON back

{
"ProcessorId": "0001",
"IssuerId": "0002",
"TransactionId": "0003",
"StepupRequestId": "0004",
"StepupType": "OTP",
"Language": "",
"Status": "SUCCESS",
"Reason": {
"ReasonCode": "",
"ReasonDescription": ""
},
"Error": {
"ReferenceNumber": "",
"Description": "",
"Message": ""
},
"Credentials": [
{
"Id": "1",
"Type": "OTPSMS",
"Text": "5012187149"
},
{
"Id": "2",
"Type": "OTPEMAIL",
"Text": "sbyrd@eeft.com"
}
]
}


instead of this one:
{
"ProcessorId": "0001",
"IssuerId": "0002",
"TransactionId": "0003",
"StepupRequestId": "0004",
"StepupType": "OTP",
"Language": "",
"Status": "SUCCESS",
"Credentials": [
{
"Id": "1",
"Type": "OTPSMS",
"Text": "5012187149"
},
{
"Id": "2",
"Type": "OTPEMAIL",
"Text": "sbyrd@eeft.com"
}
]
}


Unfortunately it isn't my specification that I'm writing to, it's someone elses.