LIBKML: only write style/style.kml if it the style table is non null, and assign an id to the document in case there's no style in the table

git-svn-id: https://svn.osgeo.org/gdal/trunk@26945 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx