Postman variable returning incorrect value

New to APIs/Postman
1

in the test for a Get by Id request I am doing this on an int? datatype property:
var r = pm.response.json();
pm.environment.set(“initialValue”, r.positionTypeID);

often r.positionTypeID returns null:
{
…
“positionTypeID” : null
…
}

in a Patch request:
{
…
“positionTypeID”: {{initialValue}},
…
}

my request fails because the patch request is:
{
…
“positionTypeID”: “”,
…
}

instead of “positionTypeID”: null,

How do I get the variable “positionTypeID” to return null instead of “”?