[8.18](backport #44576) [metricbeat][kafka] fix panic when fetching consumergroup member assignments

[mergify]

Proposed commit message

[metricbeat][kafka] fix panic when fetching consumergroup member assignments.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Closes Panic in metricbeat with kafka module metric set "consumergroup" #44397

Use cases

Screenshots

Logs

---

This is an automatic backport of pull request #44576 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 91de405 has failed:

On branch mergify/bp/8.18/pr-44576
Your branch is up to date with 'origin/8.18'.

You are currently cherry-picking commit 91de405ef.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   CHANGELOG.next.asciidoc
	modified:   metricbeat/module/kafka/broker.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   metricbeat/module/kafka/broker_test.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally