Add the possibility to get an empty EventStream without call getEventStream

The only solution to have a new EventStream is to call getEventStream from eventstore. Sometimes we need a new empty EventStream. we can create an empty EventStream outside of eventStore and reference the eventStore but it's cleaner to encapsulate this possibility in the eventStore.

I've create an getNewEventStream function that return an empty eventStream initialized with steamId and current eventStore.