chore: Simplify code slightly

Author: xmatthias

freqtrade/persistence/trade_model.py

@@ -1190,11 +1190,10 @@ def calc_profit_ratio(
         if (amount is None) and (open_rate is None):
             open_trade_value = self.open_trade_value
         else:
-            if amount is None:
-                amount = self.amount
-            if open_rate is None:
-                open_rate = self.open_rate
-            open_trade_value = self._calc_open_trade_value(amount, open_rate)
+            # Fall back to trade.amount and self.open_rate if necessary
+            open_trade_value = self._calc_open_trade_value(
+                amount or self.amount, open_rate or self.open_rate
+            )
 
         if open_trade_value == 0.0:
             return 0.0