BUG: cannot print max_acceleration_power_on_time

- This happens when we use EmptyMotors
- ValueError: attempt to get argmax of an empty sequence

rocketpy/simulation/flight.py

@@ -2137,6 +2137,9 @@ def max_acceleration_power_on_time(self):
         burn_out_time_index = find_closest(
             self.acceleration.source[:, 0], self.rocket.motor.burn_out_time
         )
+        if burn_out_time_index == 0:
+            return 0 # the burn out time is before the first time step
+
         max_acceleration_time_index = np.argmax(
             self.acceleration[:burn_out_time_index, 1]
         )