[nicfit] do keepalive without iq:last but just send " "

good because google talk (and other servers) do not support iq:last
good because we do not disconnect ourselves if time changes
good because we do not disconnect ourselves on modal dialogs
good because even stpeter thinks that is the best way to do keepalives
good because we do not flood the server with iq:last
bad because I say it is good :D