sync-diff-inspector: fix error when json column is NULL

[Leavrth]

What problem does this PR solve?

Issue Number: close #736

What is changed and how it works?

check whether the json column is NULL to avoid json unmarshal error.

Check List

Tests

• Unit test
• Integration test

Release Note

None

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: 3pointer

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[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2023-08-21 09:21:57.678707384 +0000 UTC m=+1141282.227723356: :ballot_box_with_check: agreed by 3pointer.

[Leavrth]

/merge

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[Leavrth]

/tide