OGRLayer: Identity(), Union() and Intersect(): avoid creating features with null or empty geometry; when intersecting 2 polygons, do not consider a point or linear intersection as a valid one (#4772)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@25427 f0d54148-0727-0410-94bb-9a71ac55c965