[JSON SUPPORT] Select nested field return exception in JDBC format

[penghuo]

Issue

Test Data

PUT /customer/_doc/1
{
  "name": "John Doe",
  "address": {
    "city": "seattle",
    "state": "wa",
    "street": {
      "zip": 98004
    }
  }
}

Query

GET _opendistro/_sql/
{
  "query": "SELECT address.street.zip FROM customer"
}

Result

{
  "error": {
    "reason": "There was internal problem at backend",
    "details": "",
    "type": "NullPointerException"
  },
  "status": 500
}

Expectation

{
  "schema": [{
    "name": "address.city.zip",
    "type": "integer"
  }],
  "total": 1,
  "datarows": [[98004]],
  "size": 1,
  "status": 200
}