Null list<struct<...>> is written and read as an empty list

### Apache Iceberg version

main (development)

### Please describe the bug 🐞

A null `list>` is silently rebuilt as an **empty** list. This is known on
the read path — `tests/integration/test_reads.py::test_null_list_and_map` currently
asserts the corrupted value, with the correct assertion commented out pending
apache/arrow#38809:

```python
# This should be:
# assert arrow_table["col_list_with_struct"].to_pylist() == [None, [{'test': 1}]]
# Once https://github.com/apache/arrow/issues/38809 has been fixed
assert arrow_table["col_list_with_struct"].to_pylist() == [[], [{"test": 1}]]
```

Two things seem worth reporting on top of that.

**It also affects the write path**, where the consequence is worse. The Parquet file
pyiceberg writes contains an empty list, so the null is gone at rest and no reader —
pyiceberg, Spark, Trino — can recover it. On read the file is at least still correct.

**It does not depend on the upstream Arrow fix.** `pa.LargeListArray.from_arrays`
takes a `mask` argument — since well before pyiceberg's `pyarrow>=18.0.0` floor — so
this particular null loss can be fixed independently of apache/arrow#38809, which is
still open.

This is the `array>` case from #251. That issue was closed in March 2025 on
the strength of this test existing, but the assertion it makes is the corrupted one;
the `array` case in the issue body was genuinely fixed by #252, while the
`array>` case in the issue *title* — which @HonahX flagged as
remaining broken in
https://github.com/apache/iceberg-python/pull/252#discussion_r1467065763 — was not.

#### Reproduction (write path)

pyiceberg 0.11.1, pyarrow 25.0.1:

```python
import os, shutil, glob
import pyarrow as pa, pyarrow.parquet as pq
from pyiceberg.catalog.sql import SqlCatalog

WH = "/tmp/wh"; shutil.rmtree(WH, ignore_errors=True); os.makedirs(WH)

sch = pa.schema([
pa.field("id", pa.int32(), nullable=False),
pa.field("l_struct", pa.list_(pa.field("element", pa.struct([pa.field("x", pa.int32())]), nullable=True)), nullable=True),
pa.field("l_int", pa.list_(pa.field("element", pa.int32(), nullable=True)), nullable=True),
])
tbl = pa.table({"id": [1, 2, 3, 4],
"l_struct": [[{"x": 1}], [], None, [{"x": 3}]],
"l_int": [[1], [], None, [3]]}, schema=sch)

cat = SqlCatalog("r", uri=f"sqlite:///{WH}/c.db", warehouse=f"file://{WH}")
cat.create_namespace("ns")
it = cat.create_table("ns.t", schema=tbl.schema)
it.append(tbl)

out = it.scan().to_arrow()
for c in ("l_struct", "l_int"):
print(f"{c:9s} in={tbl.column(c).to_pylist()!s:35s} out={out.column(c).to_pylist()}")

# the loss is already in the file on disk, not in the read path
f = glob.glob(f"{WH}/**/*.parquet", recursive=True)[0]
print("raw parquet:", pq.read_table(f).column("l_struct").to_pylist())
```

Output:

```
l_struct in=[[{'x': 1}], [], None, [{'x': 3}]] out=[[{'x': 1}], [], [], [{'x': 3}]]
l_int in=[[1], [], None, [3]] out=[[1], [], None, [3]]
raw parquet: [[{'x': 1}], [], [], [{'x': 3}]]
```

`l_int` round-trips correctly, and writing the same `pa.Table` with `pq.write_table`
preserves the null, so the loss is not pyarrow's.

#### Cause

`ArrowProjectionVisitor.list` rebuilds the array when the element is a struct
(`pyiceberg/io/pyarrow.py:2078` on `main` @ 7539661):

```python
if isinstance(value_array, pa.StructArray):
# This can be removed once this has been fixed:
# https://github.com/apache/arrow/issues/38809
list_array = pa.LargeListArray.from_arrays(list_array.offsets, value_array)
```

`from_arrays` receives the offsets buffer alone, which cannot express a null list, and
no mask, so the validity bitmap is dropped. That is also why only this one shape is
affected: the `struct` visitor passes `mask=struct_array.is_null()`, the `map` visitor
does not rebuild at all, and a list whose element is a primitive never enters this
branch. The visitor runs on both paths, which is why the same root cause shows up as
the read-side assertion above and as the write-side corruption here.

#### Fix

Carrying the mask over is enough:

```python
list_array = pa.LargeListArray.from_arrays(list_array.offsets, value_array, mask=list_array.is_null())
```

With that change the reproduction above returns `None` for both columns, and
`test_null_list_and_map` passes with its commented-out assertion restored. I have not
looked at int32-offset or sliced-array handling of this call, which the existing line
already relies on; that appears independent of the mask.

I have this on a branch with a unit test covering the write path and the integration
assertion un-commented, and can open a PR.

Found while testing a third-party Iceberg writer against pyiceberg as a reader.

### Willingness to contribute

- [x] I can contribute a fix for this bug independently

Start at pyiceberg/io/pyarrow.py:2078 in ArrowProjectionVisitor.list and inspect tests/integration/test_reads.py::test_null_list_and_map. Verify the reproduction preserves null struct lists in the written Parquet file and on read, then run the focused integration test and the relevant unit test; done means the restored assertion passes without regressing primitive-list handling.

Scritto dal modello di indicizzazione a partire dal testo della issue.