Simplify leverage_prep interface

freqtrade/persistence/models.py

@@ -425,11 +425,13 @@ class LocalTrade():
             self.stop_loss_pct = -1 * abs(percent)
         self.stoploss_last_update = datetime.utcnow()
 
-    def set_isolated_liq(self, isolated_liq: float):
+    def set_isolated_liq(self, isolated_liq: Optional[float]):
         """
         Method you should use to set self.liquidation price.
         Assures stop_loss is not passed the liquidation price
         """
+        if not isolated_liq:
+            return
         if self.stop_loss is not None:
             if self.is_short:
                 self.stop_loss = min(self.stop_loss, isolated_liq)