How to retrieve a "Null" facet from the @search.facets result?

Question

I've got a faceted field which value is not required, so some records will have it assigned to null.

I am able to search and get all the records that have it set to null by using

$filter=country eq null

However, @検索君 .facets will only return the values that are assigned:

"country": [
{
"count": 43,
"value": "Brazil"
},
{
"count": 120,
"value": "Canada"
}
]

Could I get a response like that?
"country": [
{
"count": 43,
"value": "Brazil"
},
{
"count": 120,
"value": "Canada"
},
{
"count": 350,
"value": null
},
]