Spark `xxhash64` ignores validity buffer

[neilconway]

Describe the bug

Similar to #25419 and #25293

To Reproduce

CREATE TABLE t AS SELECT named_struct('a', 1, 'b', 'x') AS s;

-- nullif only clears the struct's validity bit; the child arrays still hold 1 and 'x'
SELECT s IS NULL AS is_null,
       xxhash64(s) AS masked_null,
       xxhash64(arrow_cast(NULL, 'Struct("a": Int64, "b": Utf8)')) AS plain_null
FROM (SELECT nullif(s, s) AS s FROM t);

$ cargo run --bin datafusion-cli -- --spark -f repro.sql

+---------+---------------------+------------+
| is_null | masked_null         | plain_null |
+---------+---------------------+------------+
| true    | 6158280145716684778 | 42         |
+---------+---------------------+------------+

Expected behavior

No response

Additional context

No response