NPE fix for when toolbar invisible and menubar visible.

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@33107 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
Using Giac's eliminate() when myeliminate returns with an empty string, fixes 44forum.ggb in #4221 (backport for 4.4 needed)

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@33108 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e