**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+
else
        {
            if ((arguments.size() != 1 && arguments.size() != 2) || (to_decimal && arguments.size() != 2))
                throw Exception("Number of arguments for function " + getName() + " doesn't match: passed " + toString(arguments.size()) +
                    ", should be 1 or 2. Second argument only make sense for DateTime (time zone, optional) and Decimal (scale).",
                    ErrorCodes::NUMBER_OF_ARGUMENTS_DOESNT_MATCH);
            if (!isStringOrFixedString(arguments[0].type))
            {
                if (this->getName().find("OrZero") != std::string::npos ||
                    this->getName().find("OrNull") != std::string::npos)
                    throw Exception("Illegal type " + arguments[0].type->getName() + " of first argument of function " + getName() +
                            ". Conversion functions with postfix 'OrZero' or 'OrNull'  should take String argument",
                            ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);
                else
                    throw Exception("Illegal type " + arguments[0].type->getName() + " of first argument of function " + getName(),
                            ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);
            }
            if (arguments.size() == 2)
            {
                if constexpr (std::is_same_v<ToDataType, DataTypeDateTime>)
                {
                    if (!isString(arguments[1].type))
                        throw Exception("Illegal type " + arguments[1].type->getName() + " of 2nd argument of function " + getName(),
                            ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);
                }
                else if constexpr (to_decimal)
                {
                    if (!isInteger(arguments[1].type))
                        throw Exception("Illegal type " + arguments[1].type->getName() + " of 2nd argument of function " + getName(),
                            ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);