[BUG] Field with null value is omitted when write back to result index

[seankao-az]

What is the bug?
When writing back query result to query_execution_result_{datasource}, fields with null value is dropped. I encountered this with cloudwatch datasource.

Sample query:

SELECT srcPort FROM mycw.default.FLE_GLE_VPC_LoadGen LIMIT 10

Raw result:

+-------+
|srcPort|
+-------+
|  20641|
|  20641|
|   null|
|   null|
|  20641|
|  20641|
|   null|
|   null|
|  20641|
|  20641|
+-------+

Corresponding doc that is written back to query_execution_result_{datasource}

{
    "_index": "query_execution_result_mycw",
    "_id": "-BgYq4sBhRhnt4SbZiGq",
    "_score": 1,
    "_source": {
      "result": [
        "{'srcPort':20641}",
        "{'srcPort':20641}",
        "{}",
        "{}",
        "{'srcPort':20641}",
        "{'srcPort':20641}",
        "{}",
        "{}",
        "{'srcPort':20641}",
        "{'srcPort':20641}"
      ],
      "schema": [
        "{'column_name':'srcPort','data_type':'long'}"
      ],
      "jobRunId": "00feineujeco7g0a",
      "applicationId": "00feggo87noppi09",
      "dataSourceName": "mycw",
      "status": "SUCCESS",
      "error": "",
      "queryId": "cEZnRDVhVlVjWG15Y3c=",
      "queryText": "select srcPort from mycw.default.FLE_GLE_VPC_LoadGen limit 10",
      "sessionId": "dThKam9xTU5sVW15Y3c=",
      "updateTime": 1699382583256,
      "queryRunTime": 11805
    }
}

Expected result:

"result": [
        "{'srcPort':20641}",
        "{'srcPort':20641}",
        "{'srcPort':null}",
        "{'srcPort':null}",
        "{'srcPort':20641}",
        "{'srcPort':20641}",
        "{'srcPort':null}",
        "{'srcPort':null}",
        "{'srcPort':20641}",
        "{'srcPort':20641}"
      ]