Shape driver handling of empty geometries

[Even Rouault]

This ticket is to track the behaviour of the shape driver regarding to how it handles empty/degenerated geometries when reading a shapefile.

In branches/1.5, in r13749, I commited a fix to restore GDAL 1.4.X behaviour that returns an empty OGRMultiPolygon when psShape->nParts == 0 for a SHPT_POLYGON/SHPT_POLYGONM/SHPT_POLYGONZ. In GDAL 1.5.0, when GEOS was available, the OGRGeometryFactory::organizePolygons returned a NULL pointer that was not corretly handled and caused a crash.

I trunk, in r13762, we return a NULL pointer for empty multipoints, lines and polygons instead of empty OGRMultiPoint, OGRMultiLineString and OGRMultiPolygon.

[Even Rouault]

In trunk, in r13764, I've added a test to ogr_shape.py to check that empty multipoints, multilines and multipolygones are returned as features with NULL geometries.