Issue description
Example usecase of a bad behaviour:
Too many trucks at ore mine tower, being loaded right now or waiting to unload "cannot deliver, no valid destination".
I unassign a truck at ore mine tower, then assign a truck e.g. at a coal mine tower.
Observed: Usually a truck fully loaded with ore gets assigned (probably because it was idle, waiting for a delivery job), goes straight to the coal mine tower. Only to still be blocked until it finally can unload at the iron furnace. for which it has to go back all the way and then has to drive to the coal mine again.
Expected behavior
Trucks should be empty before actually driving to the new assignment location.
As long as they fail to find a valid delivery destination they should just stay where they are.
Game version
v0.4.8