Feature::field should return Result<Option<FieldValue>>

[dead10ck]

Currently, Feature::field returns Result<FieldValue>, but fields can be null. When this is the case, the default value for the FieldValue variant is returned. This makes it difficult to distinguish e.g. a legitimate Integer 0 value from the absence of a value.

I am not very familiar with the GDAL lib, but just taking a quick one through, it looks like maybe the OGR_F_IsFieldNull function could be used to see if a field is null first, and if so, return None.