修复重试文件没有清楚的问题

src/DotXxlJob.Core/Queue/CallbackTaskQueue.cs

@@ -89,7 +89,7 @@ namespace DotXxlJob.Core.Queue
             ReturnT result; 
             try
             {
-                result = await this._adminClient.Callback(list);
+                result = await _adminClient.Callback(list);
             }
             catch (Exception ex){
                 this._logger.LogError(ex,"trigger callback error:{error}",ex.Message);

src/DotXxlJob.Core/Queue/RetryCallbackTaskQueue.cs

@@ -76,6 +76,15 @@ namespace DotXxlJob.Core.Queue
                 }
             }
 
+            try
+            {
+                File.Delete(_backupFile); //删除备份文件
+            }
+
+            catch (Exception ex)
+            {
+                this._logger.LogError(ex, "delete backup file  error:{error}", ex.Message);
+            }
             if (list.Any())
             {
                 foreach (var item in list)
@@ -88,7 +97,7 @@ namespace DotXxlJob.Core.Queue
 
         public void Push(List<HandleCallbackParam> list)
         {
-            if (!list.Any())
+            if (list?.Count == 0)
             {
                 return;
             }