var groupsWithoutMembers = [];
var list = new GlideRecord("sys_user_group");
list.addEncodedQuery("active=true^RLQUERYsys_user_grmember.group,=0,m2m^ENDRLQUERY");
list.query();
while (list.next()) {
    groupsWithoutMembers.push(list.sys_id + "");
}

var taskGR = new GlideRecord("task");
taskGR.addQuery("sys_class_name", "incident");
taskGR.addQuery("assignment_groupIN" + groupsWithoutMembers);
taskGR.query();