修复用户本位币为空字符串时的默认值逻辑，兼容null和空两种情况

AssetManager.Services/PortfolioService.cs

@@ -198,7 +198,7 @@ public class PortfolioService : IPortfolioService
             throw new Exception("User not found");
         }
 
-        string targetCurrency = user.DefaultCurrency ?? "CNY";
+        string targetCurrency = !string.IsNullOrEmpty(user.DefaultCurrency) ? user.DefaultCurrency : "CNY";
         _logger.LogInformation("用户 {UserId} 默认本位币: {Currency}", userId, targetCurrency);
         decimal totalValueInTargetCurrency = 0;
         decimal totalCostInTargetCurrency = 0;