Updating User Records to clear values to Null

Does anyone know how to enter a null value into a user field using the API?

This adds a space to the user record’s firstName field.

{
"firstName": " ",
    "lastName": "Smith"
}

This does not update the field at all.

{
"firstName": "",
    "lastName": "Smith"
}

This too does not update the field at all.

{
"firstName": null,
    "lastName": "Smith"
}

 And this makes the first name actually read [null]

{
"firstName": "null",
    "lastName": "Smith"
}