[Bug](Executor) ARRAY_UNION with empty left array and nested-array right input reaches BE and fails with Unexpected columns

## Description
`ARRAY_UNION` fails on the backend when the left argument is an empty array and the right argument contains nested arrays. The expression reaches the BE and then reports an error like `Unexpected columns`.

## Discussion Summary
- The issue occurs when `ARRAY_UNION` is used with an empty left array and a nested-array right input.
- Related functions already support nested arrays; for example, `array_distinct(array_concat([[[1], [2]]], ...))` returns nested arrays.
- Extending the BE implementation of `ARRAY_UNION` to properly handle nested-array inputs is preferable to adding FE-side restrictions.

## Expected Behavior
`ARRAY_UNION` should either return a clean result for nested-array inputs or be rejected cleanly at the FE/BE boundary if unsupported. The current failure happens inside the BE, which is not user-friendly.

Start by tracing ARRAY_UNION across the FE/BE boundary, focusing on the backend path that handles an empty left array and nested-array right input. Reproduce the Unexpected columns failure, then verify that nested-array input either returns the expected result or is rejected cleanly before reaching the backend.

Written by the indexing model from the issue text.