{
  "NullValue": {
    "type": "string",
    "nullable": true,
    "not": {
      "anyOf": [
        {"type": "string"},
        {"type": "number"},
        {"type": "boolean"},
        {"type": "object"},
        {
          "type": "array",
          "items": {}
        }
      ]
    }
  }
}