NotificationTool: do not add empty Subject:; To: or From: fields

We might want to rely on MailHost for filling those fields.

product/ERP5/Tool/NotificationTool.py

@@ -134,10 +134,13 @@ def buildEmailMessage(from_url, to_url, msg=None,
     for key, value in additional_headers.items():
       message.add_header(key, value)
 
-  message.add_header('Subject',
-                      make_header([(subject, 'utf-8')]).encode())
-  message.add_header('From', from_url)
-  message.add_header('To', to_url)
+  if subject:
+    message.add_header('Subject',
+                        make_header([(subject, 'utf-8')]).encode())
+  if from_url:
+    message.add_header('From', from_url)
+  if to_url:
+    message.add_header('To', to_url)
 
   for attachment in attachment_list:
     attachment_name = attachment.get('name', '')