Make GoToPartyLeader fall back to the leader's current cell

[John Snail]

Currently, GoToPartyLeader sets the variable used for SystemLongDistanceMoveTo to null, meaning followers are left behind if a cell cannot be found for them. This leads to other bugs such as them becoming hostile when the leader’s zone unloads.

The variable should instead be set to TargetCell, so followers will be put in the same cell as their leader as a failsafe, and the null check for that variable should be removed (the cells are already checked earlier in the method).