title: [NSKeyedUnarchiver initForReadingWithData:]: data is NULL
body: <p>Among the settings that I am saving to NSUserDefaults is a non-mandatory object that doesn't make sense to have an out-of-the-box default.  Until the user sets a value for this object, the app generates the error "[NSKeyedUnarchiver initForReadingWithData:]: data is NULL" when I unarchive from NSUserDefaults.  I am ignoring the error and the app works fine otherwise.</p>
  
  <p>Is there a best practice to avoid this, and is this a worry?</p>