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

system/user

@@ -1273,20 +1273,24 @@ class AIX(User):
 
         if self.groups is not None:
             current_groups = self.user_group_membership()
-            groups = self.get_groups_set()
-            group_diff = set(current_groups).symmetric_difference(groups)
             groups_need_mod = False
+            groups = []
 
-            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:
+            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)
+
+                if group_diff:
+                    if self.append:
+                        for g in groups:
+                            if g in group_diff:
+                                groups_need_mod = True
+                                break
+                    else:
+                        groups_need_mod = True
 
             if groups_need_mod:
                 cmd.append('-G')