Allow to configure serialization strategy for unset features

[erdalkaraca]

Currently, when unset features are serialized, this will lead a null value at the JavaScript/JSON side which leads to lot of boilerplate null checking code.
It would be great to allow how unset features should be treated.

For example, for a multi-valued feature, I would like the JSON object to have an empty array instead of null, so I do not have to check in JavaScript for null before accessing it.

[eneufeld]

We gladly accept a PR for this.