All-null column get wrong parquet null-counts

[crepererum]

Describe the bug
When serializing an all-null arrow array to parquet, the null-count in the stats is always 0.

To Reproduce
Steps to reproduce the behavior:

#[test]
fn statistics_null_counts_only_nulls() {
    // check that null-count statistics for "only NULL"-columns are correct
    let values = Arc::new(UInt64Array::from(vec![
        None,
        None,
    ]));
    let file = one_column_roundtrip("null_counts", values, true);

    // check statistics are valid
    let reader = SerializedFileReader::new(file).unwrap();
    let metadata = reader.metadata();
    assert_eq!(metadata.num_row_groups(), 1);
    let row_group = metadata.row_group(0);
    assert_eq!(row_group.num_columns(), 1);
    let column = row_group.column(0);
    let stats = column.statistics().unwrap();
    assert_eq!(stats.null_count(), 2);  // <<< this fails, null count is 0
}

Expected behavior
For all-null columns the null-count should be the same as the number of rows.

Additional context
Tested on c863a2c44bffa5c092a49e07910d5e9225483193.

I am claiming this issue since I have a fix ready.