Coerce undefined values to their correct zero type, per proto3 presence spec

[dpup]

Following up from #5 the gateway will omit zero values unless emit_unpopulated is set to true. Even then, there may be cases where a client expects a field that isn't sent by the server.

It would be nice for the model layer to handle coercion to the zero-type, so application code doesn't need to explicitly check for undefined.