[Parquet][C++] Logical types with sort order UNKNOWN are missing null_count statistics

### Describe the enhancement requested

The C++ Parquet implementation after adding variant and geometry will have several logical types with a sort order of UNKNOWN. The current implementation of statistics will not calculate a null count and add that statistic to the column metadata if the sort order is unknown, so this particular piece of information will be missing for geometry, geography, and variant. For geometry in particular, it will be needed to effectively push down a query rectangle (or else there is no mechanism to detect completely null row groups).

I'm not sure what the best way is to implement this...for geometry specifically we could keep track of the null count in the `GeoStatistics` but this wouldn't help with variant. I'm also not sure if the null count + statistics should be written at the page level for these types or not.

Noted by @wgtmac in https://github.com/apache/arrow/pull/45459

### Component(s)

Parquet, C++