kafka(ticdc): fix kafka sink panic when kill the downstream kafka (#8526)

[ti-chi-bot]

This is an automated cherry-pick of #8526

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]

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[3AceShowHand]

/merge

[ti-chi-bot]

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

Commit hash: 9694aa6

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (release-6.5@9eb0b15). Click here to learn what that means.
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❗ Current head b229a7c differs from pull request most recent head 9694aa6. Consider uploading reports for the commit 9694aa6 to get more accurate results

Additional details and impacted files

Flag	Coverage Δ
cdc	67.7859% <0.0000%> (?)
dm	55.0692% <0.0000%> (?)
engine	66.7429% <0.0000%> (?)

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