[BUG] NPE on bulk upsert request

[vladru]

What is the bug?

Bulk upsert request fails with HTTP status code 500 and null pointer exception with reason
"Cannot invoke "Object.toString()" because the return value of "java.util.Map.get(Object)" is null"

How can one reproduce the bug?

Use simplest bulk upsert request, for an example from the API documentation:

POST _bulk
{ "update": { "_index": "movies", "_id": "tt0816711" } }
{ "doc" : { "title": "World War Z" }, "doc_as_upsert": true }

The request fail as described above.
I've tried send such request using javascript client (v.2.12.0) and also from DevTools in OpenSearch Dshboard - responses are the same

What is the expected behavior?

Request inserts new record to the index.

What is your host/environment?

Client side Windows 11 Enterprise
Domain cluster OpenSearch 2.15 (latest)

Do you have any screenshots?

No

Do you have any additional context?

No

[vladru]

I think it was mustake create this bug report in this project, so I've created new one in
opensearch-project/OpenSearch#16524

[dblock]

Thanks, let's close this one here.