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

[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-8.1@8dd1329). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.1     #55068   +/-   ##
================================================
  Coverage               ?   71.1600%           
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  Files                  ?       1465           
  Lines                  ?     422317           
  Branches               ?          0           
================================================
  Hits                   ?     300521           
  Misses                 ?     101297           
  Partials               ?      20499           

Flag	Coverage Δ
unit	71.1600% <88.8888%> (?)

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

Components	Coverage Δ
dumpling	53.9957% <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	41.2164% <0.0000%> (?)

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tangenta, wjhuang2016

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-02 08:31:33.571485305 +0000 UTC m=+78290.790246913: ☑️ agreed by tangenta.
• 2024-08-02 08:32:20.403301194 +0000 UTC m=+78337.622062802: ☑️ agreed by wjhuang2016.