fix: FetchForWritng() async projection returns null if there are no additional events to apply

[elexisvenator]

When applying events to update an async projection snapshot, existing behavior returns null instead of the snapshot if there are no events to apply.
From codegen:

        public override GroupProjection Build(IReadOnlyList<IEvent> events, IQuerySession session, GroupProjection snapshot)
        {
            if (!events.Any()) return null;  // This is the problem, there are no additional events to apply, should return snapshot
            GroupProjection groupProjection = null; // this also appears to be a no-op
            
            // ...

            return snapshot;
        }

Testing this has been a headache - I am clearly missing something when it comes to properly testing generated code. In theory the included test matches my applications code but does not reproduce the issue. Additionally the following other bugs are identified:

• Calling RebuildProjection on a projection whose type is a nested class does not work due to the way the type name is compared to the registered projections.
• The projection randomly is not populated, with the 'Name' property being null. But this only happens 25% of the time - feels like async projection stuff happening in the background but I thought RebuildProjection would block until the rebuild is done.