Do not set empty To, Cc nor Bcc in email message

Since gh-102988, email.utils.getaddresses() return [('', '')] for invalid email
addresses which includes empty string if the header is set with an empty list.

Closes #13649

modules/account_dunning_email/account.py

@@ -149,9 +149,12 @@ class Dunning(metaclass=PoolMeta):
             if dunning.level.email_from:
                 from_ = dunning.level.email_from
         set_from_header(msg, sender, from_)
-        msg['To'] = to
-        msg['Cc'] = cc
-        msg['Bcc'] = bcc
+        if to:
+            msg['To'] = to
+        if cc:
+            msg['Cc'] = cc
+        if bcc:
+            msg['Bcc'] = bcc
         msg['Subject'] = title
         msg['Auto-Submitted'] = 'auto-generated'
         return msg

modules/notification_email/notification.py

@@ -200,9 +200,12 @@ class Email(ModelSQL, ModelView):
                     filename=('utf-8', '', name))
 
         set_from_header(msg, sender, from_)
-        msg['To'] = to
-        msg['Cc'] = cc
-        msg['Bcc'] = bcc
+        if to:
+            msg['To'] = to
+        if cc:
+            msg['Cc'] = cc
+        if bcc:
+            msg['Bcc'] = bcc
         msg['Subject'] = title
         msg['Auto-Submitted'] = 'auto-generated'
         return msg

modules/notification_email/tests/test_module.py

@@ -120,8 +120,8 @@ class NotificationEmailTestCase(CompanyTestMixin, ModuleTestCase):
 
         email, = Email.search([])
         self.assertEqual(email.recipients, 'Administrator <user@example.com>')
-        self.assertEqual(email.recipients_secondary, '')
-        self.assertEqual(email.recipients_hidden, '')
+        self.assertEqual(email.recipients_secondary, None)
+        self.assertEqual(email.recipients_hidden, None)
         self.assertEqual(email.subject, 'Notification Email')
         self.assertEqual(email.resource, user)
         self.assertEqual(email.notification_email, notification_email)

trytond/trytond/ir/email_.py

@@ -153,9 +153,12 @@ class Email(ResourceAccessMixin, ModelSQL, ModelView):
                     filename=('utf-8', '', name))
         from_ = config.get('email', 'from')
         set_from_header(msg, from_, user.email or from_)
-        msg['To'] = [format_address(a, n) for n, a in getaddresses([to])]
-        msg['Cc'] = [format_address(a, n) for n, a in getaddresses([cc])]
-        msg['Bcc'] = [format_address(a, n) for n, a in getaddresses([bcc])]
+        if to:
+            msg['To'] = [format_address(a, n) for n, a in getaddresses([to])]
+        if cc:
+            msg['Cc'] = [format_address(a, n) for n, a in getaddresses([cc])]
+        if bcc:
+            msg['Bcc'] = [format_address(a, n) for n, a in getaddresses([bcc])]
         msg['Subject'] = subject
 
         send_message_transactional(msg, strict=True)