C API: make GDALSetDefaultRAT() accept a NULL RAT. All drivers are ready for that (i.e. return CE_Failure), except HFA (so fix it). PAM already accepts unsetting a RAT

git-svn-id: https://svn.osgeo.org/gdal/trunk@25815 f0d54148-0727-0410-94bb-9a71ac55c965