异步获取网页时 返回值修改为null...

CTZLauncher/HttpHelper.cs

@@ -76,7 +76,7 @@ namespace CityCraft
                 while (readyState != HttpReadyState.完成) { this.DoEvent(); }
                 return responseBody;
             }
-            return string.Empty;
+            return null;
         }
 
         /// <summary>