LIBKML: transform multigeometry with one single component into single geometry; emit a warning when an empty multigeometry is written (ATC 66)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26952 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx