Test treatment of optional values #8
Description
In grpc all fields are optional and get a default value when not set, see https://developers.google.com/protocol-buffers/docs/proto3#default
For example the default value for a string type is an empty string.
In Xenon null is used a lot as the default value.
Mapping default values between Xenon and gprc should be tested.
Giving Xenon an empty string instead of null could cause unintended behavior.