`nw.Enum([])` raises when evaluating `__{repr,hash}__`

[dangotbanned]

Discovered in (#3233) while porting some tests from polars.

This one is py-polars/tests/unit/test_selectors.py#L943-L956

Repro

import narwhals as nw
nw.Enum([])

File narwhals/narwhals/dtypes.py:697, in Enum.__repr__(self)
    696 def __repr__(self) -> str:
--> [697]     return f"{type(self).__name__}(categories={list(self.categories)!r})"

File narwhals/narwhals/dtypes.py:667, in Enum.categories(self)
    665     return self._cached_categories
    666 msg = f"Internal structure of {type(self).__name__!r} is invalid."  # pragma: no cover
--> [667] raise TypeError(msg)

TypeError: Internal structure of 'Enum' is invalid.

The good news though (I guess) is it doesn't seem to get triggered when we started with pl.Enum([]):

import polars as pl

native = pl.Series([]).cast(pl.Enum)
series = nw.from_native(native, series_only=True)
series.dtype

Enum(categories=[])

Having this working isn't personally of interest, but I saw the # pragma: no cover and thought it was worth showing how we can get there 😄