kvclient: filter the old value if old-value is disabled (#1304)

[ti-srebot]

cherry-pick #1304 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In ticdc repo:
git pr https://github.com/pingcap/ticdc/pull/1347

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/ticdc.git pr/1347:release-4.0-6a7d5ce31565

---

What problem does this PR solve?

fix #1299

What is changed and how it works?

set Oldvalue to nil when old-value is disabled

Check List

Tests

• Unit test
• Integration test

Release note

• Fix the old-value switch of a changefeed could be affected by another changefeed

[ti-srebot]

/run-all-tests

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (release-4.0@f7db19c). Click here to learn what that means.
The diff coverage is n/a.

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[amyangfei]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

@ti-srebot merge failed.

[amyangfei]

/merge

[ti-srebot]

/run-all-tests