Fix: pathfinders always tried to avoid docking tiles (even if nothing was on them) (#9522)

When coming across any docking tile (for example, all tiles around
an oilrig are docking tiles), it always at least added a penalty
of 3 times a normal tile, even when there are no ships on them.

In result, the pathfinder got suggested to always go around docking
tiles. This was most likely not the intention of the change made in
31db4f8d5e.