JSON mode: Dropping key-value pairs with null value from fine-tuned model

Hello everyone
I have fine-tuned ChatGPT to complete a large predefined JSON form according to a given text. the problem is that the JSON form contains a lot of items the answer to some of which are not always available in the given text.
I provided the training set containing the whole JSON form in which the value of unavailable keys are null.
Now I wish such key-value pairs to be filtered out of the model’s output otherwise I would be billed for missing values.

for example I provided the training data as following

Bob is an data scientist and he is working at Google

{
"first name": "Bob",
"last name": null,
"profession": "data scientist",
"age": null,
"company": "google",
"department": null,
"university degree": null,
"phone number": null
}

now I wish the output to be as following where the null key-value pairs are filtered out:

{
"first name": "Bob",
"profession": "data scientist",
"company": "google"
}

anyway I can achieve this?