[BUG] PPL query executed on an object field with dynamic mapping disabled returns an empty result

[penghuo]

What is the bug?
A PPL query executed on an object field with dynamic mapping disabled returns an empty result. PPL supports OpenSearch object fields with dynamic mapping, where an object field is mapped to a struct type field. If a sub-field does not exist, references to that sub-field are resolved as missing values.

How can one reproduce the bug?

### create index, profile field is object type, dynamic disabled.
PUT {{baseUrl}}/test001
Content-Type: application/x-ndjson

{
  "mappings": {
    "properties": {
      "profile": {
        "type": "object",
        "dynamic": false
      }
    }
  }
}

### add data
POST {{baseUrl}}/_bulk/
Content-Type: application/x-ndjson

{"index": {"_index": "test001"}}
{"profile": {"age": 1}}
{"index": {"_index": "test001"}}
{"profile": {"address": "a"}}

### query
POST {{baseUrl}}/_plugins/_ppl
Content-Type: application/x-ndjson

{
  "query": "source = test001 "
}

### return empty result.
{
  "schema": [
    {
      "name": "profile",
      "type": "struct"
    }
  ],
  "datarows": [
    [
      {}
    ],
    [
      {}
    ]
  ],
  "total": 2,
  "size": 2
}

What is the expected behavior?
The expected a new data type

• alt1, Dynamic data type
• alt2, Any data type
• alt3, JSON data type.