List all the directions of the trains that have empty cars.
empty cars refers to load_num = 0
SELECT T2.direction FROM cars AS T1 INNER JOIN trains AS T2 ON T1.train_id = T2.id WHERE T1.load_num = 0
database name:db_id trains
cars table cars Cols: id dtype integer, train id dtype integer, position dtype integer, shape dtype text, length dtype text, sides dtype text, roof dtype text, wheels dtype integer, load_shape dtype text, load number dtype integer, 
trains table trains Cols: id dtype integer, direction dtype text, 
