**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 (!skip_not_null_check)
                {
                    const auto & col = arguments[0].column;
                    const auto & nullable_col = assert_cast<const ColumnNullable &>(*col);
                    const auto & null_map = nullable_col.getNullMapData();
                    if (!memoryIsZero(null_map.data(), 0, null_map.size()))
                        throw Exception{"Cannot convert NULL value to non-Nullable type",
                                        ErrorCodes::CANNOT_INSERT_NULL_IN_ORDINARY_COLUMN};
                }
                const ColumnNullable * nullable_source = typeid_cast<const ColumnNullable *>(arguments.front().column.get());
                return wrapper(tmp_args, nested_type, nullable_source, input_rows_count);
            };
        }
        else
            return wrapper;
    }
    
    
    WrapperType prepareImpl(const DataTypePtr & from_type, const DataTypePtr & to_type, bool requested_result_is_nullable) const
    {
        if (isUInt8(from_type) && isBool(to_type))
            return createUInt8ToBoolWrapper(from_type, to_type);
        
        bool safe_convert_custom_types = true;
        if (const auto * to_type_custom_name = to_type->getCustomName())
            safe_convert_custom_types = from_type->getCustomName() && from_type->getCustomName()->getName() == to_type_custom_name->getName();
        else if (const auto * from_type_custom_name = from_type->getCustomName())