WALK: avoid returning NULL in GetFIDColumn() and GetGeometryColumn()

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26214 f0d54148-0727-0410-94bb-9a71ac55c965
test_ogrsf: check GetFIDColumn() and GetGeometryColumn()

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26215 f0d54148-0727-0410-94bb-9a71ac55c965