fix: Pass/Retrieve empty categories from nw.Enum

[camriddell]

What type of PR is this? (check all applicable)

• 💾 Refactor
• ✨ Feature
• 🐛 Bug Fix
• 🔧 Optimization
• 📝 Documentation
• ✅ Test
• 🐳 Other

Related issues

• Closes nw.Enum([]) raises when evaluating __{repr,hash}__ #3235

Checklist

• Code follows style guide (ruff)
• Tests added
• Documented the changes
  • This was expected, but untested behavior.

If you have comments or can explain your changes, please do so below

>>> import narwhals as nw
>>> dtype = nw.Enum([])
>>> dtype.categories # old behavior: TypeError: Internal structure of 'Enum' is invalid.
()                   # new behavior: returns correctly

[dangotbanned]

Thanks @camriddell!

[MarcoGorelli]

nice, thanks!