ddl: fix the wrong logic to assert whether table has foreign key

[YangKeao]

What problem does this PR solve?

Issue Number: close #51807, close #51762

Problem Summary:

If the foreign key is an empty slice, the condition nt.ForeignKeys != nil will meet. It would be better to assert it with len(nt.ForeignKeys) > 0

What changed and how does it work?

Change nt.ForeignKeys != nil to len(nt.ForeignKeys) > 0.

Actually, I wonder whether there are other similar issues in TiDB. It's very likely to have bugs in similar pattern 🤔 .

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

None

[codecov]

Codecov Report

Merging #51808 (e17d64e) into master (5b8185f) will increase coverage by 2.0828%.
Report is 8 commits behind head on master.
The diff coverage is 100.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #51808        +/-   ##
================================================
+ Coverage   70.7345%   72.8173%   +2.0827%     
================================================
  Files          1476       1476                
  Lines        437888     437926        +38     
================================================
+ Hits         309738     318886      +9148     
+ Misses       108742      98980      -9762     
- Partials      19408      20060       +652     

Flag	Coverage Δ
integration	48.8771% <100.0000%> (?)
unit	70.4976% <0.0000%> (-0.0454%)	⬇️

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

Components	Coverage Δ
dumpling	53.9957% <ø> (ø)
parser	∅ <ø> (∅)
br	51.4616% <ø> (+5.5449%)	⬆️

[bb7133]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bb7133, ywqzzy

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• OWNERS [bb7133,ywqzzy]
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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-03-15 05:51:33.824154429 +0000 UTC m=+1010320.846400818: ☑️ agreed by ywqzzy.
• 2024-03-15 05:54:29.382235539 +0000 UTC m=+1010496.404481926: ☑️ agreed by bb7133.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #51821.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #51822.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #51831.