Fix object reference not set to an instance of an object for http proxy

framework/src/Volo.Abp.Http.Client/Volo/Abp/Http/Client/DynamicProxying/UrlBuilder.cs

@@ -139,7 +139,7 @@ namespace Volo.Abp.Http.Client.DynamicProxying
                 return dateTimeValue.ToUniversalTime().ToString("O");
             }
 
-            return value.ToString();
+            return value?.ToString();
         }
     }
 }