non-existent oss-objects should return null

aspnet-core/modules/oss-management/LINGYUN.Abp.BlobStoring.OssManagement/LINGYUN/Abp/BlobStoring/OssManagement/OssManagementBlobProvider.cs

@@ -73,7 +73,7 @@ namespace LINGYUN.Abp.BlobStoring.OssManagement
                 Logger.LogWarning("An error occurred while getting the OSS object and an empty data stream will be returned");
                 Logger.LogWarning(ex.Message);
 
-                return Stream.Null;
+                return null;
             }
         }
 

aspnet-core/modules/workflow/LINGYUN.Abp.WorkflowCore.Components/LINGYUN/Abp/WorkflowCore/Components/Primitives/SendEmail.cs

@@ -74,7 +74,7 @@ namespace LINGYUN.Abp.WorkflowCore.Components.Primitives
                 foreach (var attachment in attachments)
                 {
                     var stream = await _blobContainer.GetOrNullAsync(attachment);
-                    if (stream != null)
+                    if (stream != null && stream != Stream.Null)
                     {
                         mailMessage.Attachments.Add(new Attachment(stream, Path.GetFileName(attachment)));
                     }

aspnet-core/services/LY.MicroService.WorkflowManagement.HttpApi.Host/appsettings.Development.json

@@ -46,7 +46,7 @@
         "HostName": "127.0.0.1",
         "Port": 5672,
         "UserName": "admin",
-        "Password": "662874",
+        "Password": "123456",
         "VirtualHost": "/"
       }
     },