[C++][Python] Add default data type for completely null arrays when field is not populated in pyarrow.csv.ConvertOptions

### Describe the enhancement requested

I receive regularly updated csv files from a Government source. Sometimes a particular field is not populated in a supply.

I'd like to store as parquet and keep schemas consistent.

As a workaround, I check the schemas and case null arrays as strings, but it would be good to have something like:

```null_values_type = pa.String()```

Added to ConvertOptions. Thank you.

### Component(s)

Python