Fix #8340: Prevent null reference exception in SearchResponse<T>

[sadeghpour]

• Added null checks for HitsMetadata in SearchResponse<T>.Documents
• Ensured HitsMetadata and Hits are properly validated before accessing them
• Prevented potential crashes when the response contains no hits

Closes #8340

[cla-checker-service]

💚 CLA has been signed

[sadeghpour]

Hi team, I have signed the CLA, but the check still seems to be failing. Could you please verify? Let me know if I need to take any further action. Thanks!

[pquentin]

Hello, and thank you for your contribution! I have no idea about the code itself. I can confirm that you signed the CLA check correctly, but the issue is that your commit does not use an email address. See https://github.com/elastic/elasticsearch-net/commit/4a91b26f1095bd82681c825b441960f2dba0df07.patch.

[flobernd]

Thanks for the PR @sadeghpour !

Besides this fix, I would like to investigate the case a little bit more in depth. Would it be possible for you to send me the JSON request and response that triggers this behavior?

In our specification, both, ResponseBody<TDocument>.hits (this is HitsMetadata in the .NET client) and HitsMetadata<T>.hits are not marked as optional. I wonder if this should be changed cc @pquentin @l-trotta

[l-trotta]

@flobernd never found any evidence that hits could be null, is there a special case where it happens?

[sadeghpour]

@l-trotta You are right! After further investigation following @flobernd previous comment, I also couldn't find any evidence that hits could be null. It seems there's no special case where this happens.