fix: 修复回测统计计算空列表异常

- 修复 calculateStatistics 在 trades 为空时的 IndexOutOfBoundsException
- 添加空列表检查，避免访问 trades[0]
- 确保回测任务即使没有交易记录也能正常完成

backend/src/main/kotlin/com/wrbug/polymarketbot/service/backtest/BacktestExecutionService.kt

@@ -590,7 +590,11 @@ class BacktestExecutionService(
         var maxLoss = BigDecimal.ZERO
 
         // 计算最大回撤
-        var runningBalance = trades[0]?.balanceAfter?.toSafeBigDecimal() ?: BigDecimal.ZERO
+        var runningBalance = if (trades.isNotEmpty()) {
+            trades[0].balanceAfter?.toSafeBigDecimal() ?: BigDecimal.ZERO
+        } else {
+            BigDecimal.ZERO
+        }
         var peakBalance = runningBalance
         var maxDrawdown = BigDecimal.ZERO