**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[1].column)
                        throw Exception("Second argument for function " + getName() + " must be constant", ErrorCodes::ILLEGAL_COLUMN);
                }
                else
                {
                    throw Exception("Number of arguments for function " + getName() + " doesn't match: passed "
                        + toString(arguments.size()) + ", should be 1. Second argument makes sense only for DateTime and Decimal.",
                        ErrorCodes::NUMBER_OF_ARGUMENTS_DOESNT_MATCH);
                }
            }
            if constexpr (std::is_same_v<ToDataType, DataTypeDateTime>)
                res = std::make_shared<DataTypeDateTime>(extractTimeZoneNameFromFunctionArguments(arguments, 1, 0));
            else if constexpr (std::is_same_v<ToDataType, DataTypeDateTime64>)
                throw Exception("LOGICAL ERROR: It is a bug.", ErrorCodes::LOGICAL_ERROR);
            else if constexpr (to_decimal)
            {
                UInt64 scale = extractToDecimalScale(arguments[1]);
                res = createDecimalMaxPrecision<typename ToDataType::FieldType>(scale);
                if (!res)
                    throw Exception("Something wrong with toDecimalNNOrZero() or toDecimalNNOrNull()", ErrorCodes::LOGICAL_ERROR);
            }
            else
                res = std::make_shared<ToDataType>();
        }
        if constexpr (exception_mode == ConvertFromStringExceptionMode::Null)
            res = std::make_shared<DataTypeNullable>(res);
        return res;
    }
    template <typename ConvertToDataType>
    ColumnPtr executeInternal(const ColumnsWithTypeAndName & arguments, const DataTypePtr & result_type, size_t input_rows_count, UInt32 scale = 0) const