[Python] get_partition_keys segfaults when the partition expression is None

### Describe the bug, including details regarding any error messages, version, and platform.

`pyarrow.dataset.get_partition_keys` takes an `Expression` but does not reject
`None`, so passing it dereferences a null pointer and terminates the interpreter
rather than raising.

```python
import pyarrow.dataset as ds
ds.get_partition_keys(None)
```

```console
$ python -c "import pyarrow.dataset as ds; ds.get_partition_keys(None)"; echo "exit=$?"
Bus error: 10
exit=138
```

The same call through the private alias `ds._get_partition_keys(None)` crashes
identically.

This is the same class of problem as GH-51042 and GH-51043, where a required
Arrow wrapper argument was not declared `not None` at the Cython boundary. A
sweep of the remaining Cython bindings for required wrapper-typed parameters
without `not None` found this as the one still reachable from a public entry
point.

Expected behavior is a `TypeError`, consistent with other typed PyArrow APIs.

PyArrow 25.0.1, CPython 3.11, macOS arm64.

### Component(s)

Python