title: Set Default Display Name for all existing users that dont have display name
body: <p>does anyone know of way I can set the display name for all users that has not set display name?.by now if users has not set display name, its showing the username in any where. and i just dont want to show usernames as user display name. so please give me a code that can set defult display name for users who has not display name(empity displayname)
  Thanks in advance</p>