Add support for marshalling null elements in collections/arrays

So far, protostream has minimal support for nulls. A top level null object is marshalled as the empty (zero bytes) message.

For the sake of backward compatibility we'll keep this behavior for top level, but we need a different approach for elements of an array or collection. In this case we need to use a null token. A new field will be added in WappedMessage to represent the null.