sink: fix codec json encoding with non binary string (#2764)

[ti-chi-bot]

This is an automated cherry-pick of #2764

What problem does this PR solve?

Close #2758

Note we still don't known in which scenario the column value is string instead of []byte, need more investigation.

What is changed and how it works?

The column value type of mysql.TypeString, mysql.TypeVarString, mysql.TypeVarchar could be either string or []byte, we should consider both of these two types when converting a row column value to json encoding.

Check List

Tests

• Unit test
• Integration test

Release note

Fix json encoding could panic when processing a string type value in some cases.

[ti-chi-bot]

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[amyangfei]

/merge

[ti-chi-bot]

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

Commit hash: 28f5d9f

[codecov-commenter]

Codecov Report

Merging #2781 (28f5d9f) into release-4.0 (753073a) will increase coverage by 1.4653%.
The diff coverage is 58.8659%.

@@                 Coverage Diff                 @@
##           release-4.0      #2781        +/-   ##
===================================================
+ Coverage      53.7661%   55.2314%   +1.4652%     
===================================================
  Files              153        164        +11     
  Lines            15958      19746      +3788     
===================================================
+ Hits              8580      10906      +2326     
- Misses            6475       7805      +1330     
- Partials           903       1035       +132