fix(pg-cdc): fix postgres.publication_exist, found in issues#18628

[KeXiangWang]

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What's changed and what's your intention?

As titled

Checklist

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• I have added necessary unit tests and integration tests
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[cyliu0]

/test

[cyliu0]

/test

[cyliu0]

https://buildkite.com/risingwave-test/sysbench-cdc/builds/681#01921e48-8a30-405c-801c-9de0e1140a87

The test passed @KeXiangWang

[KeXiangWang]

Thanks @cyliu0