kafka(ticdc): fix kafka sink panic when kill the downstream kafka

[3AceShowHand]

What problem does this PR solve?

Issue Number: close #8523

What is changed and how it works?

• check admin client is not nil before close it.

Check List

Tests

• Manual test (add detailed scripts or steps below)

Questions

Will it cause performance regression or break compatibility?

Do you need to update user documentation, design documentation or monitoring documentation?

Release note

Fix cdc server panic when all Kafka servers are unavailable

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• nongfushanquan
• overvenus

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[3AceShowHand]

/run-all-tests

[sdojjy]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 41e9f01

[ti-chi-bot]

@3AceShowHand: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

trigger some heavy tests which will not run always when PR updated.

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.

[3AceShowHand]

/run-verify-ci

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.6: #8527.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #8528.