`array_union` correctness issue

Describe the bug

Error executing SQL 'SELECT array_union(a, b) FROM test_array_union'

== Results ==
!== Correct Answer - 5 ==               == Spark Answer - 5 ==
 struct<array_union(a, b):array<int>>   struct<array_union(a, b):array<int>>
 [ArrayBuffer(1)]                       [ArrayBuffer(1)]
 [ArrayBuffer(1, 2)]                    [ArrayBuffer(1, 2)]
 [ArrayBuffer(1, 2, 3, 4, 5)]           [ArrayBuffer(1, 2, 3, 4, 5)]
![ArrayBuffer(1, null, 2)]              [ArrayBuffer(null, 1, 2)]
 [null]                                 [null]

Steps to reproduce

No response

Expected behavior

No response

Additional context

No response