Ok to ONLY accept empty values with empty: True?

[peterdesmet]

For consistency, I've updated the documentation to ONLY accept empty values if empty: True is added, even when you intuitively might think otherwise: https://github.com/inbo/whip/blob/5f658fcb8adaa798ff7f7d4e2f866b89722a25ea/docs/syntax.md#empty

@stijnvanhoey does this make sense for you or is it better to allow intuition, but advise against it? 😄