Fix issue 3908. There was some ilegal operations over the sets

system/user

@@ -1273,16 +1273,20 @@ class AIX(User):
 
         if self.groups is not None:
             current_groups = self.user_group_membership()
+            groups_need_mod = False
+            groups = []
+
+            if self.groups == '':
+                if current_groups and not self.append:
+                    groups_need_mod = True
+            else:
                 groups = self.get_groups_set()
                 group_diff = set(current_groups).symmetric_difference(groups)
-            groups_need_mod = False
 
                 if group_diff:
                     if self.append:
                         for g in groups:
                             if g in group_diff:
-                            groups.extend(current_groups)
-                            set(groups)
                                 groups_need_mod = True
                                 break
                     else: