MNT: add structural mass ratio property to EmptyMotor class

Author: Gui-FernandesBR

rocketpy/motors/empty_motor.py

@@ -72,3 +72,7 @@ def propellant_I_23(self):
     @property
     def propellant_I_33(self):
         return Function(0, "Time (s)", "Propellant I_33 (kg m²)")
+
+    @property
+    def structural_mass_ratio(self):
+        return 0

rocketpy/motors/motor.py

@@ -1162,6 +1162,7 @@ def all_info(self):
         self.prints.all()
         self.plots.all()
 
+
 # TODO: move this class to a separate file, needs a breaking change warning
 class GenericMotor(Motor):
     """Class that represents a simple motor defined mainly by its thrust curve.