[FIX] mail : Remove double signature on invoice email

Current behavior :
When sending an invoice by email there was 2 signature in the mail

Steps to reproduce :
Create an invoice
Send it by email
Check the mail, there are 2 signatures

opw-2703272

closes odoo/odoo#81086

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/mail/data/mail_data.xml

@@ -269,7 +269,7 @@
                     <t t-esc="access_name"/>
                 </a>
             </div>
-            <t t-if="record.user_id and not record.env.user._is_superuser()">
+            <t t-if="record.user_id and not record.env.user._is_superuser() and signature != ''">
                 <div style="margin: 0px; padding: 0px; font-size:13px;">
                     Best regards,
                 </div>

addons/test_mail/models/test_mail_models.py

@@ -41,6 +41,11 @@ class MailTestStandard(models.Model):
     container_id = fields.Many2one('mail.test.container', tracking=True)
     company_id = fields.Many2one('res.company')
 
+    def _get_share_url(self, redirect, signup_partner, share_token):
+        """This function is required for a test on 'mail.mail_notification_paynow' template (test_message_post/test_mail_add_signature),
+        another model should be created in master"""
+        return '/mail/view'
+
 
 class MailTestActivity(models.Model):
     """ This model can be used to test activities in addition to simple chatter

addons/test_mail/tests/test_message_post.py

@@ -36,6 +36,31 @@ class TestMessagePost(TestMailCommon, TestRecipients):
         messageId = call_kw(self.env['mail.channel'], 'message_post', [test_channel.id], {'body': 'test'})
         self.assertTrue(isinstance(messageId, int))
 
+    @users('employee')
+    def test_notify_mail_add_signature(self):
+        self.test_track = self.env['mail.test.track'].with_context(self._test_context).with_user(self.user_employee).create({
+            'name': 'Test',
+            'email_from': 'ignasse@example.com'
+        })
+        self.test_track.user_id = self.env.user
+
+        signature = self.env.user.signature
+
+        template = self.env.ref('mail.mail_notification_paynow', raise_if_not_found=True).sudo()
+        self.assertIn("record.user_id.sudo().signature", template.arch)
+
+        with self.mock_mail_gateway():
+            self.test_track.message_post(body="Test body", mail_auto_delete=False, add_sign=True, partner_ids=[self.partner_1.id, self.partner_2.id], email_layout_xmlid="mail.mail_notification_paynow")
+        found_mail = self._new_mails
+        self.assertIn(signature, found_mail.body_html)
+        self.assertEqual(found_mail.body_html.count(signature), 1)
+
+        with self.mock_mail_gateway():
+            self.test_track.message_post(body="Test body", mail_auto_delete=False, add_sign=False, partner_ids=[self.partner_1.id, self.partner_2.id], email_layout_xmlid="mail.mail_notification_paynow")
+        found_mail = self._new_mails
+        self.assertNotIn(signature, found_mail.body_html)
+        self.assertEqual(found_mail.body_html.count(signature), 0)
+
     @users('employee')
     def test_notify_prepare_template_context_company_value(self):
         """ Verify that the template context company value is right