[C++][Parquet] MinMax statistics for strings may be inaccurate after a merge

### Describe the bug, including details regarding any error messages, version, and platform.

TypedStatistics::Megre function disregards column chunks when the minimum value is an empty string (represented as {null, 0}) due to the following code:
```c++
optional> CleanStatistic(
std::pair min_max, LogicalType::Type::type) {
if (min_max.first.ptr == nullptr || min_max.second.ptr == nullptr) {
return ::std::nullopt;
}
return min_max;
}
```
so both minimum and maximum values will be lost.

### Component(s)

C++