[FIX] account: send and print invoice without attachments

This traceback raises when user tries to send an invoice without attachments.

To reproduce this issue:

1) Install 'Accounting'
2) Open any existing 'Invoice'
   Note:- Invoice should not be in 'Draft'
3) Click 'Send & Print '  button an wizard will be opened.
4) Click on 'Send & Print' button of wizard.
5) Now repeat the step 3
6) This time delete the Invoice Attachment and click on 'Send & Print' button
   of wizard.

Error: 'A traceback appears': 'can only concatenate list (not "bool") to list'

On '_get_mail_params' method 'attachment_ids' is getting
values by concatenating  'mail_attachments_widget' and
'invoice_extra_attachments_data'.

See:-
https://github.com/odoo/odoo/blob/27c1384e1209339158f56c82bebf99a5b64811f4/addons/account/wizard/account_move_send.py#L434-L448

Because of user delete the 'Invoice attachments' in wizard, 'mail_attachments_widget'
will return false and it leads to above traceback.

Sentry-4305060652

closes odoo/odoo#133953

X-original-commit: f08da6fdfa881ec3e80d6fc14dddaa8df3c529ed
Signed-off-by: William André (wan) <wan@odoo.com>

addons/account/wizard/account_move_send.py

@@ -457,7 +457,7 @@ class AccountMoveSend(models.Model):
         # We must ensure the newly created PDF are added. At this point, the PDF has been generated but not added
         # to 'mail_attachments_widget'.
         seen_attachment_ids = set()
-        for attachment_data in self._get_invoice_extra_attachments_data(self.move_ids) + self.mail_attachments_widget:
+        for attachment_data in self._get_invoice_extra_attachments_data(self.move_ids) + (self.mail_attachments_widget or []):
             try:
                 attachment_id = int(attachment_data['id'])
             except ValueError: