[BUG] SUM of all-null column returns 0.0 instead of NULL per SQL standard

[penghuo]

Query Information

PPL Command/Query:

source=bounty-types | stats sum(num_field) as total_sum

Expected Result:
When all values are null, sum should return null.

Actual Result:
Returns 0 with HTTP 200.

Dataset Information

Dataset/Schema Type

• OpenTelemetry (OTEL)
• Simple Schema for Observability (SS4O)
• Open Cybersecurity Schema Framework (OCSF)
• Custom (details below)

Index Mapping

{
  "mappings": {
    "properties": {
      "num_field": { "type": "integer" }
    }
  }
}

Sample Data

[
  { "num_field": null },
  { "num_field": null },
  { "num_field": null }
]

Bug Description

Issue Summary:
SUM on all-null column returns 0 instead of null.

Steps to Reproduce:

1. Create index with nullable numeric field.
2. Insert only null values for that field.
3. Run the query above.
4. Observe total_sum = 0.

Impact:
Aggregation semantics are incorrect and may hide missing-data conditions.

[penghuo]

This is opensearch issue. opensearch-project/OpenSearch#17581.