Always reproduced:

```
milovidov-desktop :) create table table_map (a Map(String, String), b String) engine = MergeTree() order by a;

CREATE TABLE table_map
(
    `a` Map(String, String),
    `b` String
)
ENGINE = MergeTree
ORDER BY a

Query id: 8f8120c1-c165-452a-815f-2bcae3957e2a

Ok.

0 rows in set. Elapsed: 0.002 sec. 

milovidov-desktop :) SELECT b FROM table_map WHERE a = map('name', NULL, '', NULL)

SELECT b
FROM table_map
WHERE a = {'name', NULL, '', NULL}
```
{
        if (!isStringOrFixedString(from_type))
            throw Exception{"CAST AS FixedString is only implemented for types String and FixedString", ErrorCodes::NOT_IMPLEMENTED};
        bool exception_mode_null = cast_type == CastType::accurateOrNull;
        return [exception_mode_null, N] (ColumnsWithTypeAndName & arguments, const DataTypePtr &, const ColumnNullable *, size_t )
        {
            if (exception_mode_null)
                return FunctionToFixedString::executeForN<ConvertToFixedStringExceptionMode::Null>(arguments, N);
            else
                return FunctionToFixedString::executeForN<ConvertToFixedStringExceptionMode::Throw>(arguments, N);
        };
    }
    template <typename ToDataType>
    std::enable_if_t<IsDataTypeDecimal<ToDataType>, WrapperType>
    createDecimalWrapper(const DataTypePtr & from_type, const ToDataType * to_type, bool requested_result_is_nullable) const
    {
        TypeIndex type_index = from_type->getTypeId();
        UInt32 scale = to_type->getScale();
        WhichDataType which(type_index);
        bool ok = which.isNativeInt() || which.isNativeUInt() || which.isDecimal() || which.isFloat() || which.isDateOrDateTime()
            || which.isStringOrFixedString();
        if (!ok)
        {
            if (cast_type == CastType::accurateOrNull)
                return createToNullableColumnWrapper();
            else
                throw Exception{"Conversion from " + from_type->getName() + " to " + to_type->getName() + " is not supported",
                    ErrorCodes::CANNOT_CONVERT_TYPE};
        }