[Iris] DestroyObjectsPendingDestroy not called on server with no clients

In UNetDriver::TickFlush, UReplicationSystem::PreSendUpdate is not called on the server when it does not have any client connections. PreSendUpdate is where DestroyObjectsPendingDestroy is called from, so running a server without any clients can cause the number of NetObjects to continually increase. When these actors/objects are destroyed, their NetObjects will not get destroyed or free their internal index, resulting in the server hitting the fatal error: "NetRefHandleManager: Hit the maximum limit of active replicated objects..."
This can especially be an issue for listen servers, which may be more likely to run for some time without remote clients connecting.

On the server, continually create and destroy replicated actors/objects.
Run the server for some time without connecting any clients.
After enough replicated objects have been created/destroyed, a fatal error will occur due to the max limit of active replicated objects being hit.