use empty Arrays to express emptiness about swift-fhir

[p2]

There's a few reasons why I'm not a fan of this change.

One is consistency – all properties are optional to enable the json: initializers.

Then we'd also need to decide, upon serialization, what to do – never show empty arrays, always show them? With both nil and empty available the dev can take care of this.

And, not least, Swift makes it really easy to deal with nil-ness and emptyness: you can use guard and test for nil-ness and emptiness in one line and you can use map() and filter() over nil and empty arrays easily.

But I'm open for discussion! And I just arrived at the DevDays, yes, so cu around!