string cast array missing null value

**Describe the bug**

**To Reproduce**

```sql
spark-sql (default)> select * from test_t1 ;
NULL
```

```sql
set spark.auron.enable=true;
select cast(array(c1) as string ) from test_t1 ;
```
```
[]
```

---

```sql
set spark.auron.enable=false;
set spark.sql.legacy.castComplexTypesToString.enabled=false;
select cast(array(c1) as string ) from test_t1 ;
```

```
[null]
```

**Expected behavior**

**Screenshots**

**Additional context**

Start by running the provided Spark SQL reproductions with spark.auron.enable set to true and false, then compare the cast(array(c1) as string) results when c1 is NULL. Trace the Auron path responsible for complex-type string casting; done means the enabled path preserves the null value as [null], matching the non-Auron result.

Written by the indexing model from the issue text.