Update the overlord solution

Better than `code.length == 0` would be `code.empty?` or `code.nil?`. If you can get the code to be nil-or-something, though, you could do something like this:

```
def set_activation_code(code)
  session[:activation_code] = validated_code(code) || '1234'
end

def validated_code(code)
  code if code =~ /\d{4}/
end