fix: null reference when ending highest amount without any shares

src/inventory_wars/scoring.py

@@ -42,8 +42,10 @@ class HighestAmount(Scoring):
         return 0
 
     def calculate_end_score(self) -> ItemShare | None:
+        if self.max:
             self.max.score = 1
             return self.max
+        return None
 
 
 class FirstThenHighest(Scoring):