ddl: fix exchange partition check on empty foreign keys (#51834)

[ti-chi-bot]

This is an automated cherry-pick of #51834

This is an automated cherry-pick of #51808

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

[ti-chi-bot]

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