cdc/codec: fix string for unsigned value in canal-json.

[3AceShowHand]

What problem does this PR solve?

Issue Number: close #4635

What is changed and how it works?

handle the case when unsigned in string

Check List

Tests

• No code

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

Release note

fix `Canal-JSON` meet `unsigned` SQL typed in `string`, which cause CDC server panic.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• maxshuang
• overvenus

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[codecov-commenter]

Codecov Report

Merging #4629 (4ff0b1f) into master (9607554) will decrease coverage by 0.1504%.
The diff coverage is 45.6942%.

Flag	Coverage Δ
cdc	59.7319% <40.2969%> (-0.1903%)	⬇️
dm	51.8975% <52.3560%> (-0.1314%)	⬇️

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

@@               Coverage Diff                @@
##             master      #4629        +/-   ##
================================================
- Coverage   55.6402%   55.4897%   -0.1505%     
================================================
  Files           494        508        +14     
  Lines         61283      63309      +2026     
================================================
+ Hits          34098      35130      +1032     
- Misses        23750      24686       +936     
- Partials       3435       3493        +58     

[3AceShowHand]

/run-all-tests

[overvenus]

/merge

[ti-chi-bot]

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

Commit hash: 4ff0b1f

[ti-chi-bot]

@3AceShowHand: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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[3AceShowHand]

/run-integration-test /tidb=pr/32434
/run-leak-test /tidb=pr/32434

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #4657.