Gracefully handle missing "modifyHDIVStateParameter" session attribute

"modifyHDIVStateParameter" may be missing if the session was initialized by HDIV 1.x. This effectively breaks upgrades from HDIV 1.x to HDIV 2.x until all sessions expire or the clients clear their cookies. Gracefully handle a missing "modifyHDIVStateParameter" instead of crashing with a NPE