Don't try to draw a polygon with no points.
Don't try to draw a polygon with no points.

Hit during an edge case of test suite.

git-svn-id: 83c4c97093aeea62ac982fbfd516bf9db27f5c99@1127 c36c8488-0289-0348-9b64-b301f74bd9a7

BlueBrain/Tuvok,BlueBrain/Tuvok,BlueBrain/Tuvok