fix: 修复 tokenUsage 为空时的空引用问题

Yi.Abp.Net8/module/ai-hub/Yi.Framework.AiHub.Domain/Managers/UsageStatisticsManager.cs

@@ -21,11 +21,11 @@ public class UsageStatisticsManager : DomainService
     public async Task SetUsageAsync(Guid? userId, string modelId, ThorUsageResponse? tokenUsage)
     {
         long inputTokenCount = tokenUsage?.PromptTokens
-                               ?? tokenUsage.InputTokens
+                               ?? tokenUsage?.InputTokens
                                ?? 0;
 
         long outputTokenCount = tokenUsage?.CompletionTokens
-                                ?? tokenUsage.OutputTokens
+                                ?? tokenUsage?.OutputTokens
                                 ?? 0;
 
         await using (await DistributedLock.AcquireLockAsync($"UsageStatistics:{userId?.ToString()}"))