ddl: fix the batch check for unique index (#40672)

[ti-chi-bot]

This is an automated cherry-pick of #40672

What problem does this PR solve?

Issue Number: close #40592, related to #40464.

Problem Summary:

Previously, I didn't consider the null values for #40464. For the unique index, the null column value keys are encoded as null_val(00)+handle and the value is empty. However, TiDB tries to decode the handle from index value, the panic happened.

What is changed and how it works?

This PR takes the null values into consideration, and refine the batch check logic by getting the original key to prevent data inconsistency.

Check List

Tests

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

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Benjamin2037
• wjhuang2016

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[tangenta]

/test mysql-test

[Benjamin2037]

LGTM

[Benjamin2037]

/merge

[ti-chi-bot]

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

Commit hash: c9b9a67

[ti-chi-bot]

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[tangenta]

/test unit-test