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

[wk989898]

What problem does this PR solve?

Issue Number: ref #11994

What is changed and how it works?

related to #11995
don't treat as a nil when the default value is the byte literal "null"

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

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]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: 3AceShowHand, asddongmen

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-01-17 07:15:24.579726748 +0000 UTC m=+275596.034772882: ☑️ agreed by 3AceShowHand.
• 2025-01-17 07:18:46.749668784 +0000 UTC m=+275798.204714933: ☑️ agreed by asddongmen.

[wk989898]

/retest

[wk989898]

/retest-required

[wk989898]

/retest-required

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.1: #12020.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #12021.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #12022.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #12023.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #12024.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #12025.