```
SELECT
    CAST(([2, 1, 1023], ['', '']), 'Map(UInt8, String)') AS map,
    map[10]
```

Triggers `Assertion 'n < size()' failed` in `DB::ColumnString::operator[](unsigned long)`
https://clickhouse-test-reports.s3.yandex.net/18481/64c4ade5b0e0c9a13d6c2475431daf431fc97554/fuzzer/fuzzer.log

Introduced here: #17829
First found here: #18481
vec_null_map_to = &col_null_map_to->getData();
        }
        const ColumnString::Chars * chars = nullptr;
        const IColumn::Offsets * offsets = nullptr;
        size_t fixed_string_size = 0;
        if constexpr (std::is_same_v<FromDataType, DataTypeString>)
        {
            chars = &col_from_string->getChars();
            offsets = &col_from_string->getOffsets();
        }
        else
        {
            chars = &col_from_fixed_string->getChars();
            fixed_string_size = col_from_fixed_string->getN();
        }
        size_t current_offset = 0;
        for (size_t i = 0; i < size; ++i)
        {
            size_t next_offset = std::is_same_v<FromDataType, DataTypeString> ? (*offsets)[i] : (current_offset + fixed_string_size);
            size_t string_size = std::is_same_v<FromDataType, DataTypeString> ? next_offset - current_offset - 1 : fixed_string_size;
            ReadBufferFromMemory read_buffer(&(*chars)[current_offset], string_size);
            if constexpr (exception_mode == ConvertFromStringExceptionMode::Throw)
            {
                if constexpr (parsing_mode == ConvertFromStringParsingMode::BestEffort)
                {
                    if constexpr (to_datetime64)
                    {
                        DateTime64 res = 0;
                        parseDateTime64BestEffort(res, vec_to.getScale(), read_buffer, *local_time_zone, *utc_time_zone);
                        vec_to[i] = res;