Added support for creating a valid stream with tick=0

So, the current Stream default arguent is 0, and that's used to test if the replay is empty. However, if we want to create an actual stream from tick 0, that breaks (i.e. scan returns [], [] instead of the first fp and the packet for tick 0). This patch just changes the default from 0 to None so that a stream at tick 0 can be created.