**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 (const auto * from_tuple = checkAndGetDataType<DataTypeTuple>(from_type_untyped.get()))
        {
            if (from_tuple->getElements().size() != 2)
                throw Exception{"CAST AS Map from tuple requeires 2 elements.\n"
                    "Left type: " + from_tuple->getName() + ", right type: " + to_type->getName(), ErrorCodes::TYPE_MISMATCH};
            DataTypes from_kv_types;
            const auto & to_kv_types = to_type->getKeyValueTypes();
            for (const auto & elem : from_tuple->getElements())
            {
                const auto * type_array = checkAndGetDataType<DataTypeArray>(elem.get());
                if (!type_array)
                    throw Exception(ErrorCodes::TYPE_MISMATCH,
                        "CAST AS Map can only be performed from tuples of array. Got: {}", from_tuple->getName());
                from_kv_types.push_back(type_array->getNestedType());