**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+
static FunctionPtr create() { return std::make_shared<FunctionConvertFromString>(); }
    String getName() const override
    {
        return name;
    }
    bool isVariadic() const override { return true; }
    bool isSuitableForShortCircuitArgumentsExecution(const DataTypesWithConstInfo & ) const override { return true; }
    size_t getNumberOfArguments() const override { return 0; }
    bool useDefaultImplementationForConstants() const override { return true; }
    bool canBeExecutedOnDefaultArguments() const override { return false; }
    ColumnNumbers getArgumentsThatAreAlwaysConstant() const override { return {1}; }
    DataTypePtr getReturnTypeImpl(const ColumnsWithTypeAndName & arguments) const override
    {
        DataTypePtr res;
        if (isDateTime64<Name, ToDataType>(arguments))
        {
            validateFunctionArgumentTypes(*this, arguments,
                FunctionArgumentDescriptors{{"string", &isStringOrFixedString<IDataType>, nullptr, "String or FixedString"}},
                
                FunctionArgumentDescriptors{
                    {"precision", &isUInt8<IDataType>, isColumnConst, "const UInt8"},
                    {"timezone", &isStringOrFixedString<IDataType>, isColumnConst, "const String or FixedString"},
                });
            UInt64 scale = to_datetime64 ? DataTypeDateTime64::default_scale : 0;
            if (arguments.size() > 1)
                scale = extractToDecimalScale(arguments[1]);
            const auto timezone = extractTimeZoneNameFromFunctionArguments(arguments, 2, 0);
            res = scale == 0 ? res = std::make_shared<DataTypeDateTime>(timezone) : std::make_shared<DataTypeDateTime64>(scale, timezone);
        }