refactor: Add None checks to Position class property methods

src/trading/portfolio.py

@@ -14,21 +14,30 @@ class Position:
     @property
     def current_value(self) -> float:
         # TODO: Implement real-time price fetching
+        if self.entry_price is None or self.shares is None:
+            return 0.0
         return self.shares * self.entry_price
 
     @property
     def potential_profit(self) -> float:
         """Calculate potential profit at target price"""
+        if self.target_price is None or self.entry_price is None or self.shares is None:
+            return 0.0
         return (self.target_price - self.entry_price) * self.shares
     
     @property
     def potential_loss(self) -> float:
         """Calculate potential loss at stop loss"""
+        if self.stop_loss is None or self.entry_price is None or self.shares is None:
+            return 0.0
         return (self.stop_loss - self.entry_price) * self.shares
     
     @property
     def risk_reward_ratio(self) -> float:
         """Calculate risk/reward ratio"""
+        if (self.target_price is None or self.entry_price is None or 
+            self.stop_loss is None):
+            return 0.0
         potential_gain = self.target_price - self.entry_price
         potential_risk = self.entry_price - self.stop_loss
         return abs(potential_gain / potential_risk) if potential_risk != 0 else 0