修改备份文件null判断

MECF.Framework.Common/MECF/Framework/Common/SCCore/SystemConfigManager.cs

@@ -146,7 +146,7 @@ namespace MECF.Framework.Common.SCCore
             var qry = from x in files
                       orderby x.LastWriteTime
                       select x;
-            string newestBackupPath = qry.Last()?.FullName;
+            string newestBackupPath = qry?.Last().FullName;
             if (!string.IsNullOrEmpty(newestBackupPath))
             {
                 var xmlDocument = new XmlDocument();