fix: add null check in RouteCache.GetRoute to prevent ArgumentNullException

- Allow nullable tenantCode and serviceName parameters
- Return null early if serviceName is null or empty
- Fix test: GetRoute_WithNullServiceName_ShouldReturnNull

src/yarpgateway/Services/RouteCache.cs

@@ -54,13 +54,23 @@ public class RouteCache : IRouteCache
         _logger.LogInformation("Route cache reloaded");
     }
 
-    public RouteInfo? GetRoute(string tenantCode, string serviceName)
+    public RouteInfo? GetRoute(string? tenantCode, string? serviceName)
     {
+        // 参数校验
+        if (string.IsNullOrEmpty(serviceName))
+        {
+            _logger.LogDebug("GetRoute called with null or empty serviceName");
+            return null;
+        }
+        
+        tenantCode ??= string.Empty;
+        
         _lock.EnterUpgradeableReadLock();
         try
         {
             // 1. 优先查找租户专用路由
-            if (_tenantRoutes.TryGetValue(tenantCode, out var tenantRouteMap) &&
+            if (!string.IsNullOrEmpty(tenantCode) &&
+                _tenantRoutes.TryGetValue(tenantCode, out var tenantRouteMap) &&
                 tenantRouteMap.TryGetValue(serviceName, out var tenantRoute))
             {
                 _logger.LogDebug("Found tenant-specific route: {Tenant}/{Service} -> {Cluster}",