Track edges sometimes have no edge group

[Vap0r1ze]

It seems to happen to random edges every time a player joins, as a result of the first received TrackGraphSyncPacket saying they are part of the NULL_GROUP group type. It also seems to only happen to edges that have a signal on them.

This is very hard for me to debug because I'm unsure what the purpose of the "null" edge group and "passive" edge group are. @simibubi If you have the time I'd appreciate if you could help me out with understanding that part of the graph code whenever u have the time, since you've written most of it.

Tested on the latest mc1.20.1/dev with the following: create_tracks.dat.

Screenshots of an example because my video uploads are failing

[Vap0r1ze]

After more debugging I think I actually understand the purpose of the null group type, and I think I've figured out the actual issue:

If TrackGraphSync::sendFullGraphTo sends the client a TrackEdge but it's points do not exist in the same packet, the client will throw away those points since it has no instance of them yet.

It seems like this would happen pretty reliably on any server with a graph over 500 nodes and a healthy amount of signals. Although I only noticed it because of #8345