**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+
UInt32 scale = extractToDecimalScale(scale_column);
                result_column = ConvertImpl<LeftDataType, RightDataType, Name, SpecialTag>::execute(arguments, result_type, input_rows_count, scale);
            }
            else if constexpr (IsDataTypeDateOrDateTime<RightDataType> && std::is_same_v<LeftDataType, DataTypeDateTime64>)
            {
                const auto * dt64 = assert_cast<const DataTypeDateTime64 *>(arguments[0].type.get());
                result_column = ConvertImpl<LeftDataType, RightDataType, Name, SpecialTag>::execute(arguments, result_type, input_rows_count, dt64->getScale());
            }
            else if constexpr (IsDataTypeDecimalOrNumber<LeftDataType> && IsDataTypeDecimalOrNumber<RightDataType>)
            {
                using LeftT = typename LeftDataType::FieldType;
                using RightT = typename RightDataType::FieldType;
                static constexpr bool bad_left =
                    is_decimal<LeftT> || std::is_floating_point_v<LeftT> || is_big_int_v<LeftT> || is_signed_v<LeftT>;
                static constexpr bool bad_right =
                    is_decimal<RightT> || std::is_floating_point_v<RightT> || is_big_int_v<RightT> || is_signed_v<RightT>;
                
                if constexpr ((bad_left && std::is_same_v<RightDataType, DataTypeUUID>) ||
                              (bad_right && std::is_same_v<LeftDataType, DataTypeUUID>))
                {
                    throw Exception("Wrong UUID conversion", ErrorCodes::CANNOT_CONVERT_TYPE);
                }
                else
                {
                    result_column
                        = ConvertImpl<LeftDataType, RightDataType, Name, SpecialTag>::execute(arguments, result_type, input_rows_count);
                }
            }
            else
            {