修正附件为null时报错的问题

MailSend.Web/Controllers/MailController.cs

@@ -102,7 +102,7 @@ namespace MailSend.Web.Controllers
                 mailSend.ShowName = item.ShowName;
 
                 //附件处理
-                if (item.Attaches.Count > 0)
+                if (item.Attaches != null && item.Attaches.Count > 0)
                 {
                     var list = new List<AttachFileModel>();
                     foreach (var att in item.Attaches)