ddl: set proper default flen of decimal(0) columns (#54028)

[ti-chi-bot]

This is an automated cherry-pick of #54028

What problem does this PR solve?

Issue Number: close #53779

Problem Summary:

See #53779.

What changed and how does it work?

Handle Decimal(0) properly in preprocessing.

Check List

Tests

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

  • I checked and no code files have been changed.

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

[GMHDBJD]

LGTM

[tangenta]

/retest

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: GMHDBJD, tangenta

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-07-15 08:53:18.416761896 +0000 UTC m=+257620.407703367: ☑️ agreed by GMHDBJD.
• 2024-07-17 09:02:45.247644836 +0000 UTC m=+430987.238586306: ☑️ agreed by tangenta.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Please upload report for BASE (release-7.5@ed1e010). Learn more about missing BASE report.

Additional details and impacted files

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Flag	Coverage Δ
unit	72.1567% <88.8888%> (?)

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Components	Coverage Δ
dumpling	53.9913% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	53.5486% <0.0000%> (?)