Verify groups string is not empty before trying add user to groups

library/system/user

@@ -279,7 +279,7 @@ class User(object):
             cmd.append('-g')
             cmd.append(self.group)
 
-        if self.groups is not None:
+        if self.groups is not None and len(self.groups):
             groups = self.get_groups_set()
             cmd.append('-G')
             cmd.append(','.join(groups))
@@ -1221,7 +1221,7 @@ class AIX(User):
             cmd.append('-g')
             cmd.append(self.group)
 
-        if self.groups is not None:
+        if self.groups is not None and len(self.groups):
             groups = self.get_groups_set()
             cmd.append('-G')
             cmd.append(','.join(groups))