[BUG] Pipeline fails to process null change event (document DB source)

[JonahCalvo]

Describe the bug
User noticed a very sharp spike in the ChangeEventsProcessingErrors metric (1.11M)

Looking into logs at the time, the following was repeatedly being output:

2025-09-29T14:07:47.565 [documentdb-source-3] ERROR org.opensearch.dataprepper.plugins.mongo.stream.StreamWorker - Failed to add records to buffer with error
java.lang.NullPointerException: Cannot invoke "org.bson.Document.toJson(org.bson.json.JsonWriterSettings)" because the return value of "com.mongodb.client.model.changestream.ChangeStreamDocument.getFullDocument()" is null

Later on after these logs had stop, the following warning was repeatedly displayed (unsure if related):

2025-09-29T14:07:50.573 [documentdb-pipeline-s3-sink-worker-4-thread-2] WARN  org.opensearch.dataprepper.plugins.sink.opensearch.BulkRetryStrategy - index = parts operation = Index, error = [66ead8422f9ba6016c4c365a]: version conflict, current version [1759154109000203] is higher or equal to the one provided [1759153858000095] |  

A circuit breaker flip was also observed.

To Reproduce
I'm not familiar enough with the ChangeStreamDocument to know if this is expected to be null or if this is a bug from the other service

Expected behavior
Null document should be ignored?

[dlvenable]

@dinujoh , @oeyh , Would one of you be able to look at this issue?