If you spot a juicy target before reaching your patrol area, by all means attack. There is no penalty for showing up in your patrol grid with empty tubes. You still get the renown after putting in your 24 hours whether you sink something there or not. In real life the U-Boats were constantly being shifted about by Bdu. They rarely had just one grid to patrol. Boats without torpedoes but with fuel still in the tanks usually ended up weather-reporting or re-fueling other boats.
