BUG: fixed __calculate_rail_button_forces() by assigning a Function(0) to null_force instead of an empty array

Author: leogrosa

rocketpy/simulation/flight.py

@@ -2954,7 +2954,7 @@ def __calculate_rail_button_forces(self):  # TODO: complex method.
             Rail Button 2 force in the 2 direction
         """
         # First check for no rail phase or rail buttons
-        null_force = []
+        null_force = Function(0)
         if self.out_of_rail_time_index == 0:  # No rail phase, no rail button forces
             warnings.warn(
                 "Trying to calculate rail button forces without a rail phase defined."