Support reading null values for numeric fields on Shapefile import via LayerMapping

[Kathryn Killebrew]

On reading a Shapefile into a model object via LayerMapping, set numeric field value to None instead of zero if the model field is nullable and input field value is either null or unset. Currently, both nulls and zeroes in the input are stored as zeroes.

Shapefiles do not officially support storing nulls:
​http://resources.esri.com/help/9.3/ArcGISDesktop/com/Gp_ToolRef/geoprocessing_tool_reference/geoprocessing_considerations_for_shapefile_output.htm

However, on inspecting the DBF contents via ogrinfo with GDAL 2.1 or 2.2, both zeroes and nulls display, so some unofficial support exists.

GDAL 2.2 added support for distinguishing between "unset" fields and those explicitly set to null:
​https://trac.osgeo.org/gdal/wiki/rfc67_nullfieldvalues

Adding null support should be possible by either checking before casting the value with OGR IsFieldSetAndNotNull on GDAL 2.2+ or checking with OGR IsFieldSet on older GDAL versions:
​https://www.gdal.org/classOGRFeature.html#a260b925400b72914eb1b8cc9a3bf4029

[Tim Graham]

​PR

[Tim Graham <timograham@…>]

In 75d6278:

Fixed #30020 -- Fixed reading nulls with LayerMapping.