This is simply how XMPP works. Groups are attached to contacts if the group has no contacts then according to XMPP this group does not exist. There maybe intuitive ways to deal with this but I would ask you this question, why do you want an empty group?
I would like to create a group (say mySchoolFriends) then want to move some id’'s which are in other group(say friends) to this newly created group(mySchoolFriends). Its like any messanger works(i.e. yahoo or google)