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.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 5 lines in your changes missing coverage. Please review.

Please upload report for BASE (release-8.1@8243402). Learn more about missing BASE report.

Additional details and impacted files

Components	Coverage Δ
cdc	61.4605% <0.0000%> (?)
dm	51.0468% <0.0000%> (?)
engine	63.4161% <0.0000%> (?)

Flag	Coverage Δ
unit	57.2636% <50.0000%> (?)

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@@               Coverage Diff                @@
##             release-8.1     #12010   +/-   ##
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  Coverage               ?   57.2636%           
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  Files                  ?        854           
  Lines                  ?     126562           
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  Hits                   ?      72474           
  Misses                 ?      48648           
  Partials               ?       5440           

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: wk989898

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[wk989898]

/retest