[BUG] TransformRunnerIT reports null_pointer_exception

[Hailong-am]

What is the bug?

org.opensearch.indexmanagement.transform.TransformRunnerIT > test continuous transform with a lot of buckets FAILED
    org.opensearch.client.ResponseException: method [PUT], host [http://[::1]:36319], URI [/_plugins/_transform/transform_index_with_lots_of_buckets?refresh=true], status line [HTTP/1.1 500 Internal Server Error]
    {"error":{"root_cause":[{"type":"null_pointer_exception","reason":"parseAggregators(xcp) must not be null"}],"type":"null_pointer_exception","reason":"parseAggregators(xcp) must not be null"},"status":500}
        at __randomizedtesting.SeedInfo.seed([D1C2DEC441108C8B:85E8E73F9A387140]:0)
        at app//org.opensearch.client.RestClient.convertResponse(RestClient.java:384)
        at app//org.opensearch.client.RestClient.performRequest(RestClient.java:354)
        at app//org.opensearch.client.RestClient.performRequest(RestClient.java:329)
        at app//org.opensearch.indexmanagement.TestHelpersKt.makeRequest(TestHelpers.kt:86)
        at app//org.opensearch.indexmanagement.TestHelpersKt.makeRequest$default(TestHelpers.kt:69)
        at app//org.opensearch.indexmanagement.transform.TransformRestTestCase.createTransformJson(TransformRestTestCase.kt:79)
        at app//org.opensearch.indexmanagement.transform.TransformRestTestCase.createTransform(TransformRestTestCase.kt:58)
        at app//org.opensearch.indexmanagement.transform.TransformRestTestCase.createTransform$default(TransformRestTestCase.kt:49)
        at app//org.opensearch.indexmanagement.transform.TransformRunnerIT.test continuous transform with a lot of buckets(TransformRunnerIT.kt:1368)

How can one reproduce the bug?
Steps to reproduce the behavior:
./gradlew integTest

What is the expected behavior?
integration test should pass

What is your host/environment?

• OS: [e.g. iOS]
• Version 2.x and main
• Plugins

Do you have any screenshots?
If applicable, add screenshots to help explain your problem.

Do you have any additional context?
Cause by this change, method parseAggregators could return null
opensearch-project/OpenSearch#8206