PPL stats command throws NullPointerException on empty index

[qianheng-aws]

Description

Running a PPL stats command on an empty index causes a NullPointerException instead of returning an empty result set.

Steps to reproduce

1. Create an empty index: PUT /empty_test
2. Run PPL: source=empty_test | stats count()
3. Get a 500 error with NullPointerException in the logs

Stack trace

java.lang.NullPointerException
    at org.opensearch.sql.expression.aggregation.CountAggregator.iterate(CountAggregator.java:45)
    at org.opensearch.sql.planner.physical.AggregationOperator.next(AggregationOperator.java:78)

Expected behavior

Should return count() = 0 for an empty index.

Environment

• OpenSearch 2.17
• SQL plugin 2.17.0