[spark] Fix writing null struct col

[Zouxxyy]

Purpose

CREATE TABLE t (i INT, s STRUCT<f1: INT, f2: INT>);
INSERT INTO t VALUES ((1, null), (2, STRUCT(null, null));
SELECT s FROM t;

before

[null,null], [null,null]

after

-- orc and avro
null, [null,null]

There is still bug with reading parquet, which fix involves core changes, add an issue to track it #4785

Tests

API and Format

Documentation

[JingsongLi]

+1