[FIX] mail: fix recipients (partner) not displayed in preview

To reproduce the problem (example):
- install the project app with demo data
- activate the debug mode
- go to Settings -> Technical -> Templates
- choose the template "Task: Rating Request"
- in email configuration, "To (partners)" is set to
${object.rating_get_partner_id().id}
- click on preview
- choose the task: "Room 2: Decoration"
- the "Recipients" field value should be "YourCompany, Joel Willis" but this
field remains empty without the fix

Technical note: partner_to of the template was not rendered which was making
generate_recipients returning incorrect values for partner_ids.

Task-3196081

Part-of: odoo/odoo#113341

addons/mail/wizard/mail_template_preview.py

@@ -68,7 +68,7 @@ class MailTemplatePreview(models.TransientModel):
             else:
                 copy_depends_values['resource_ref'] = '%s,%s' % (self.resource_ref._name, self.resource_ref.id)
                 mail_values = mail_template.with_context(template_preview_lang=self.lang).generate_email(
-                    self.resource_ref.id, self._MAIL_TEMPLATE_FIELDS)
+                    self.resource_ref.id, self._MAIL_TEMPLATE_FIELDS + ['partner_to'])
                 self._set_mail_attributes(values=mail_values)
             self.error_msg = False
         except UserError as user_error: