139.sense

PUT my_index
{
  "mappings": {
    "my_type": {
      "properties": {
        "status_code": {
          "type":       "string",
          "index":      "not_analyzed",
          "null_value": "NULL"
        }
      }
    }
  }
}

PUT my_index/my_type/1
{
  "status_code": null
}

PUT my_index/my_type/2
{
  "status_code": []
}

GET my_index/_search
{
  "query": {
    "term": {
      "status_code": "NULL"
    }
  }
}