**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 (arguments.empty())
            throw Exception{"Function " + getName() + " expects at least 1 argument",
               ErrorCodes::TOO_FEW_ARGUMENTS_FOR_FUNCTION};
        if (result_type->onlyNull())
            return result_type->createColumnConstWithDefaultValue(input_rows_count);
        const DataTypePtr from_type = removeNullable(arguments[0].type);
        ColumnPtr result_column;
        auto call = [&](const auto & types, const auto & tag) -> bool
        {
            using Types = std::decay_t<decltype(types)>;
            using LeftDataType = typename Types::LeftType;
            using RightDataType = typename Types::RightType;
            using SpecialTag = std::decay_t<decltype(tag)>;
            if constexpr (IsDataTypeDecimal<RightDataType>)
            {
                if constexpr (std::is_same_v<RightDataType, DataTypeDateTime64>)
                {
                    
                    if (arguments.size() != 2 && arguments.size() != 3)
                        throw Exception{"Function " + getName() + " expects 2 or 3 arguments for DateTime64.",
                            ErrorCodes::TOO_FEW_ARGUMENTS_FOR_FUNCTION};
                }
                else if (arguments.size() != 2)
                {
                    throw Exception{"Function " + getName() + " expects 2 arguments for Decimal.",
                        ErrorCodes::TOO_FEW_ARGUMENTS_FOR_FUNCTION};
                }
                const ColumnWithTypeAndName & scale_column = arguments[1];