BERT uses a complex type to represent "nil" as it has different interpretations on different languages.

This is a problem with bertex as nil in Elixir, it means the atom :nil but, in Erlang, it means an empty list ([]). Today, bertex parses it as an empty list which is not nil for Elixir.

@edgurgel what do you think about this? I'm inclined to think that parsing {:bert, :nil} into Elixir's nil is the right approach.