How to aggregate nested fields with null values?

Elastic Stack Elasticsearch
2 
GET test/doc/_search
{
  "size": 0,
  "query": {
    "match_all": {}
  },
  "aggs": {
    "my_agg": {
      "nested": {
        "path": "fields"
      },
      "aggs": {
        "my_filter": {
          "filter": {
            "term": {
              "fields.name": "bbb"
            }
          },
          "aggs": {
            "my_term": {
              "terms": {
                "field": "fields.value"
              }
            }
          }
        }
      }
    }
  }
}

And the response is:

  "aggregations" : {
    "my_agg" : {
      "doc_count" : 4,
      "my_filter" : {
        "doc_count" : 2,
        "my_term" : {
          "doc_count_error_upper_bound" : 0,
          "sum_other_doc_count" : 0,
          "buckets" : [
            {
              "key" : 2,
              "doc_count" : 2
            }
          ]
        }
      }
    }
  }

I want to get also:

"key" : 0 (for N/A)
"doc_count" : 1