[Transform] possible NPE in transform preview if data is empty

[hendrikmuhs]

If data is missing the composite aggregation might be null and transform preview fails with an NPE:

{
  "error": {
    "root_cause": [
      {
        "type": "null_pointer_exception",
        "reason": """Cannot invoke "org.elasticsearch.search.aggregations.bucket.composite.CompositeAggregation.getBuckets()" because "agg" is null""",
        "stack_trace": """org.elasticsearch.ElasticsearchException$1: Cannot invoke "org.elasticsearch.search.aggregations.bucket.composite.CompositeAggregation.getBuckets()" because "agg" is null
	at org.elasticsearch.server@8.7.1/org.elasticsearch.ElasticsearchException.guessRootCauses(ElasticsearchException.java:668)

caused by:

Caused by: java.lang.NullPointerException: Cannot invoke "org.elasticsearch.search.aggregations.bucket.composite.CompositeAggregation.getBuckets()" because "agg" is null
	at org.elasticsearch.xpack.transform.transforms.pivot.AggregationResultUtils.extractCompositeAggregationResults(AggregationResultUtils.java:111)
	at org.elasticsearch.xpack.transform.transforms.pivot.Pivot.extractResults(Pivot.java:142)
	at org.elasticsearch.xpack.transform.transforms.common.AbstractCompositeAggFunction.lambda$preview$0(AbstractCompositeAggFunction.java:94)
	at org.elasticsearch.server@8.7.1/org.elasticsearch.action.ActionListener$2.onResponse(ActionListener.java:167)

Fix:

Handle null at:

https://github.com/elastic/elasticsearch/blob/main/x-pack/plugin/transform/src/main/java/org/elasticsearch/xpack/transform/transforms/common/AbstractCompositeAggFunction.java#L90

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[AjeetNathawat]

hi @hendrikmuhs I have created a PR for this issue, since this looks like a straightforward null check here is the PR: #96785 . If you have any suggestions please do let me know as I'm new to open source.