GML geometry parser: accept formulations of 'MULTIPOINT EMPTY, MULTILINESTRING EMPTY, MULTIPOLYGON EMPTY and GEOMETRYCOLLECTION EMPTY that are valid GML 3 (and accepted by PostGIS)

git-svn-id: https://svn.osgeo.org/gdal/trunk@27096 f0d54148-0727-0410-94bb-9a71ac55c965