JSON reader can create null struct children on empty lists

[nevi-me]

Describe the bug

If a list of a nested struct is empty, the JSON reader should not create a null struct value.

To Reproduce

Try to write an array of below:

#[test]
fn json_list_roundtrip() {
    let json_content = r#"
    {"list": [{"ints": 1}]}
    {"list": [{}]}
    {"list": []}
    {"list": null}
    {"list": [{"ints": null}]}
    {"list": [null]}
    "#;
    let ints_struct =
        DataType::Struct(vec![Field::new("ints", DataType::Int32, true)]);
    let list_type = DataType::List(Box::new(Field::new("item", ints_struct, true)));
    let list_field = Field::new("list", list_type, true);
    let schema = Arc::new(Schema::new(vec![list_field]));
    let builder = ReaderBuilder::new().with_schema(schema).with_batch_size(64);
    let mut reader = builder.build(std::io::Cursor::new(json_content)).unwrap();

    let batch = reader.next().unwrap().unwrap();

    let list_row = batch
        .column(0)
        .as_any()
        .downcast_ref::<ListArray>()
        .unwrap();
    let values = list_row.values();
    // the  {"list": []} value gets interpreted as an empty struct, causing the length below to be 5
    assert_eq!(values.len(), 4);
    assert_eq!(values.null_count(), 1);
}

Expected behavior

Empty lists should not create an empty/null struct as this can lead to incorrect writing of data as the list's child length would disagree with the offsets.

Additional context

This relates to #704 but doesn't yet fix it