Allow null returns in ClientConnectionRunner.getFloodfillNetworkDatabaseFacade()

This makes it so that a ClientConnectionRunner.getFloodfillNetworkDatabaseFacade() will return null when getFloodfillNetworkDatabaseFacade is called and ClientConnectionRunner._floodfillNetworkDatabaseFacade is null. That happens if ClientConnectionRunner.getDestHash() is null, which is when we should be using the main netDb anyway. This only happens in LookupDestJob, and only for the first few(3-8?) lookups. IIRC a client with a null destHash and no session is exploratory. LookupDestJob simply chooses the main netDb if getDestHash is null.