fix(data): prevent panic on nil response in checker

[DEVMANISHOFFL]

What's being changed:

This PR fixes a nil pointer dereference panic in the Checker.Do method (Issue #299).

When the underlying REST connection fails, such as when it returns a 504 Gateway Timeout or 204 No Content with a nil body in certain edge cases, the responseData object can be nil. Accessing responseData.StatusCode directly led to a panic.

I added a check to ensure that responseData is not nil before accessing it.

Review checklist

• All new code is covered by tests where it is reasonable.

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

[weaviate-git-bot]

To avoid any confusion in the future about your contribution to Weaviate, we work with a Contributor License Agreement. If you agree, you can simply add a comment to this PR that you agree with the CLA so that we can merge.

beep boop - the Weaviate bot 👋🤖

PS:
Are you already a member of the Weaviate Forum?

[DEVMANISHOFFL]

I agree with the CLA.

[bevzzz]

In those cases the returned err will be non-nil, so the condition should be err != nil and not responseData == nil.