teach vortex.encode to also accept nullable arrays (#697)

This new test fails on develop:

        def test_array_with_nulls():
            a = pa.array([b'123', None])
    >       arr = vortex.encode(a)
E pyo3_runtime.PanicException: assertion failed: nulls.is_none()

It seems odd that `from_array` has a nullability parameter, but, in
principle, we could support an array which does not know its
nullability.

pyvortex/src/encode.rs

@@ -24,7 +24,8 @@ pub fn encode<'py>(obj: &Bound<'py, PyAny>) -> PyResult<Bound<'py, PyArray>> {
 
     if obj.is_instance(&pa_array)? {
         let arrow_array = ArrowArrayData::from_pyarrow_bound(obj).map(make_array)?;
-        let enc_array = Array::from_arrow(arrow_array, false);
+        let is_nullable = arrow_array.is_nullable();
+        let enc_array = Array::from_arrow(arrow_array, is_nullable);
         Bound::new(obj.py(), PyArray::new(enc_array))
     } else if obj.is_instance(&chunked_array)? {
         let chunks: Vec<Bound<PyAny>> = obj.getattr("chunks")?.extract()?;

pyvortex/test/test_array.py

@@ -8,6 +8,12 @@ def test_primitive_array_round_trip():
     assert arr.to_arrow().combine_chunks() == a
 
 
+def test_array_with_nulls():
+    a = pa.array([b"123", None])
+    arr = vortex.encode(a)
+    assert arr.to_arrow().combine_chunks() == a
+
+
 def test_varbin_array_round_trip():
     a = pa.array(["a", "b", "c"])
     arr = vortex.encode(a)