Fix loss of entered values on user forms when a validation error occurs

Steps to reproduce:

• go to the users/groups management page as administrator
• create a new user, and fill inputs expect one that's required
• submit the form
  => the validation error occurs but fields that were already filled are lost and reset to the previous value.

This is visible on edit too, when removing a value that's required.

It seems to be a theme issue since the same page without the "nxtheme:compositiion" tag behaves correctly.