lightning: fix in-correctly check for session variable

[Little-Wallace]

What problem does this PR solve?

Problem Summary:
I'm working for use tidb-backend to replace dm/loader. And I made a mistake in #28471, where I set variables without checking them.

What is changed and how it works?

I have tested it in https://github.com/pingcap/ticdc/pull/3220/files

Check List

Tests

• Integration test

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• glorv
• lance6716

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[glorv]

/hold
before comment is resolved

[glorv]

/unhold

[glorv]

/merge

[ti-chi-bot]

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

Commit hash: ad3e6fa

[ti-chi-bot]

@Little-Wallace: Your PR was out of date, I have automatically updated it for you.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

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