#4790 null and empty checks

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@38042 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
Ticket #4800: doing the same in GuiManagerD what is there in GuiManagerW seems to fix the problem

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@38043 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e