Ability to define an empty proto message via Pojo annotations

In some cases it's useful for a empty protobuf message to be sent, for example formalising an implementation type.

Currently when trying to define an empty proto message by annotating a class with just ProtoName annotation, the following error is thrown at compile time:

Error:(71, 8) java: Class org.infinispan.notifications.cachelistener.cluster.AbstractClusterListenerUtilTest.StringAppender does not have any @ProtoField annotated fields. The class should be either annotated or it should have a custom marshaller.

It's possible to workaround this by simply creating a @Protofield annotation on a class variable which is never initialised.