Corrected rolled arguments validation

Author: Bernardo-MG

dice_notation/dice.py

@@ -129,7 +129,9 @@ def roll(self):
 
         if self.quantity == 0 or self.sides == 0:
             result = 0
-        elif self.quantity and self.quantity > 0 and self.sides and self.sides > 0:
+        elif self.quantity is None and self.sides is None:
+            result = None
+        elif self.quantity > 0 and self.sides > 0:
             for x in xrange(self.quantity):
                 result += randint(1, self.sides)
         else: