Python ManifestFile.partitions panics when partition summaries are absent

### Apache Iceberg Rust version

0.10.0 and current `main` at `4532da80f3930cdbda1cf2effe05b50165da9875`.

### Describe the bug

The Python `PyManifestFile.partitions` getter unconditionally unwraps
`ManifestFile.partitions`:

https://github.com/apache/iceberg-rust/blob/4532da80f3930cdbda1cf2effe05b50165da9875/bindings/python/src/manifest.rs#L147-L155

The core field is legitimately optional:

https://github.com/apache/iceberg-rust/blob/4532da80f3930cdbda1cf2effe05b50165da9875/crates/iceberg/src/spec/manifest_list/manifest_file.rs#L94-L100

Existing core tests construct valid manifest-list entries with
`partitions: None`, serialize them as `"partitions": null`, and verify that
V1-to-V2 projection preserves `None`:

https://github.com/apache/iceberg-rust/blob/4532da80f3930cdbda1cf2effe05b50165da9875/crates/iceberg/src/spec/manifest_list/_serde.rs#L496-L521

https://github.com/apache/iceberg-rust/blob/4532da80f3930cdbda1cf2effe05b50165da9875/crates/iceberg/src/spec/manifest_list/_serde.rs#L627-L676

As a result, parsing such a manifest list succeeds, but accessing the Python
entry's `.partitions` property raises a Rust/PyO3 panic.

### To reproduce

Add a focused unit test in `bindings/python/src/manifest.rs` that constructs
`PyManifestFile` around a valid `ManifestFile` with `partitions: None`, then
calls `partitions()`.

Run:

```shell
cargo +nightly-2026-03-05 test -p pyiceberg_core_rust test_unpartitioned_manifest_partitions -- --nocapture
```

Observed consistently across two runs:

```text
panicked at bindings/python/src/manifest.rs:152:14:
called `Option::unwrap()` on a `None` value
```

A control using `partitions: Some(vec![])` passes.

### Expected behavior

The list-returning Python getter should return an empty list when partition
summaries are absent, rather than panic.

### Suggested fix

Use `unwrap_or_default()` (or equivalent optional iteration) and add a
regression test for `partitions: None`. Existing `Some` behavior should remain
unchanged.

### Willingness to contribute

I would be willing to contribute a fix for this bug with guidance from the
Iceberg community.

Start in bindings/python/src/manifest.rs at the PyManifestFile.partitions getter, then review the optional field in crates/iceberg/src/spec/manifest_list/manifest_file.rs and the existing serialization tests. Run the focused test command from the issue after adding the regression coverage. Done means a ManifestFile with partitions: None can be accessed through Python without a panic and returns an empty list, while existing Some behavior remains unchanged.

Written by the indexing model from the issue text.