fix: 移除对 Usage.TotalTokens 的空检查，始终按 multiplier 四舍五入并赋值

移除 TotalTokens 的 null 判断，避免保留 null 值，统一将其按 multiplier 四舍五入后赋为整数，防止后续使用出现空值异常。

Yi.Abp.Net8/module/ai-hub/Yi.Framework.AiHub.Domain.Shared/Dtos/OpenAi/ThorChatCompletionsResponse.cs

@@ -74,11 +74,8 @@ public record ThorChatCompletionsResponse
             this.Usage.PromptTokens =
                 (int)Math.Round((this.Usage.PromptTokens ?? 0) * multiplier);
 
-            if (this.Usage.TotalTokens is not null)
-            {
-                this.Usage.TotalTokens =
-                    (int)Math.Round((this.Usage.TotalTokens ?? 0) * multiplier);
-            }
+            this.Usage.TotalTokens =
+                (int)Math.Round((this.Usage.TotalTokens ?? 0) * multiplier);
         }
     }
 }