Good question, we should probably Ln.w(...) it and return false for now because the user will not be able to log out. Not sure why this would return null. The source doesnt seem to have changed between 2.2 and 4.x. Here's the source in the android source: 

public static AccountManager More ...get(Context context) {
  if (context == null) throw new IllegalArgumentException("context is null");
     return (AccountManager) context.getSystemService(Context.ACCOUNT_SERVICE);
}

So we will either get an illegal argument exception or we'll get the service from the context. So I feel returning false should suffice, but we should (in theory) never hit this.