Fix panics from attempts to use nil client

[trvrnrth]

PMM-10691

• [ ] Links to other linked pull requests (optional).

---

• [x] Tests passed.
• [x] Fix conflicts with target branch.
• [x] Update jira ticket description if needed.
• [x] Attach screenshots/console output to confirm new behavior to jira ticket, if applicable.

Once all checks pass and the code is ready for review, please add pmm-review-exporters team as the reviewer. That would assign people from the review team automatically. Report any issues on our Forum or Discord.

[trvrnrth]

The Handler function could probably benefit from further re-factoring to move client dependant handling into a separate function or similar, but this PR deliberately keeps the changes minimal.

[denisok]

@trvrnrth could you please update branch, we will merge