Fix for spline's export

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@29707 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
fix for #3776: setting null cell color threw NPE [needs backporting]

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@29708 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e