**Describe what's wrong**

```sql
SELECT toTypeName(s), toString(s) FROM
(
 SELECT toFixedString('',32) AS s 
 UNION ALL
 SELECT null AS s 
)
```

this query works in all versions from 19.17 to 21.8, but stop works after 21.9+ 

https://fiddle.clickhouse.com/7bd65b36-a6e6-470a-8bdc-4c98a33ff453
21.8 works

https://fiddle.clickhouse.com/9e40a424-4027-4c7f-bd41-b20935ae9129
21.9 doesn't work
```
Received exception from server (version 21.9.6):
Code: 44. DB::Exception: Received from localhost:9000. DB::Exception: Illegal column Nullable(FixedString(32)) of first argument of function toString. (ILLEGAL_COLUMN)
```

this query works in all versions

```sql
SELECT toTypeName(s), toString(s) FROM
(
 SELECT '' AS s 
 UNION ALL
 SELECT null AS s 
)
```
https://fiddle.clickhouse.com/4f810f03-1644-4eb7-b696-0341b1bbdbc4


**Does it reproduce on recent release?**
Yes, reproduce in all releases 21.9+
{
        
        if (checkAndGetDataType<DataTypeString>(from_type_untyped.get()))
        {
            return &ConvertImplGenericFromString<ColumnString>::execute;
        }
        const auto * from_type = checkAndGetDataType<DataTypeArray>(from_type_untyped.get());
        const auto * from_type_map = checkAndGetDataType<DataTypeMap>(from_type_untyped.get());
        
        if (from_type_map)
            from_type = checkAndGetDataType<DataTypeArray>(from_type_map->getNestedType().get());
        if (!from_type)
        {
            throw Exception(ErrorCodes::TYPE_MISMATCH,
                "CAST AS Array can only be performed between same-dimensional Array, Map or String types");
        }
        DataTypePtr from_nested_type = from_type->getNestedType();
        
        bool from_empty_array = isNothing(from_nested_type);
        if (from_type->getNumberOfDimensions() != to_type.getNumberOfDimensions() && !from_empty_array)
            throw Exception(ErrorCodes::TYPE_MISMATCH,
                "CAST AS Array can only be performed between same-dimensional array types");
        const DataTypePtr & to_nested_type = to_type.getNestedType();