[VL] flaky "Array Union" test

### Backend

VL (Velox)

### Bug description

2025-07-14T04:43:04.0277187Z - Array Union *** FAILED ***
2025-07-14T04:43:04.0279357Z Incorrect evaluation: array_union([b,a,c], null), actual: WrappedArray(b, a, c), expected: null (GlutenTestsTrait.scala:281)

### Gluten version

_No response_

### Spark version

None

### Spark configurations

_No response_

### System information

_No response_

### Relevant logs

```bash

```

Inspect GlutenTestsTrait.scala around line 281 and locate the flaky "Array Union" test. Run that test to reproduce the reported array_union([b,a,c], null) result, then determine whether the test is stable and its actual result consistently matches the expected result.

Written by the indexing model from the issue text.