sink(ticdc): fix incorrect encoding default value in Avro protocol (#11995)

[ti-chi-bot]

This is an automated cherry-pick of #11995

What problem does this PR solve?

Issue Number: close #11994

What is changed and how it works?

treat as a nil when the default value is the string literal "null"

Check List

Tests

• 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

 Fix incorrect encoding default value in Avro protocol.

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

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@ti-chi-bot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	51b3047	link	true	/test pull-verify

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