sink(ticdc): convert values of pre-columns properly (#8421)

[ti-chi-bot]

This is an automated cherry-pick of #8421

What problem does this PR solve?

Issue Number: close #8420

What is changed and how it works?

• Add the missing conversion step for values of per-columns in delete event.

Check List

Tests

• Unit test
• Integration test

Questions

Will it cause performance regression or break compatibility?

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

Release note

None

[ti-chi-bot]

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[zhaoxinyu]

/merge

[ti-chi-bot]

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

Commit hash: 40896e0

[nongfushanquan]

/run-all-tests

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (release-6.5@e4c2306). Click here to learn what that means.
The diff coverage is n/a.

❗ Current head 9aeabde differs from pull request most recent head 40896e0. Consider uploading reports for the commit 40896e0 to get more accurate results

Additional details and impacted files

Flag	Coverage Δ
cdc	68.0346% <0.0000%> (?)
dm	55.1782% <0.0000%> (?)
engine	66.7668% <0.0000%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

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