Fix SNS topics fetch panics on errors

[jeniawhite]

Summary of your changes

Agentless agents were having panics on fetching SNS topics.

Related Issues

• Fixes: [Asset Inventory] Cloudbeat panics if SNS topic fetching returns error #2640

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary README/documentation (if appropriate)

Introducing a new rule?

• Generate rule metadata using this script
• Add relevant unit tests
• Generate relevant rule templates using this script, and open a PR in elastic/packages/cloud_security_posture

[mergify]

This pull request does not have a backport label. Could you fix it @jeniawhite? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 8./d branch. /d is the digit
  NOTE: backport-v8.x has been added to help with the transition to the new branch 8.x.

[moukoublen]

Should we backport to 8.17 as well?

@kubasobon @oren-zohar ?

[kubasobon]

@moukoublen Yeah, I think nil-reference panic fixes merit a backport. I'm adding a label.