[FIX] mass_mailing: Fix when reply_model is Null

There exists edge case on `reply_model`. when value of
`mail_messages.model` in database is empty then `mail_messages.model`
returns False, hence `reply_model` becomes False which raises
an error because _get_id does not accept a falsy values.
(because query: `SELECT id FROM ir_model WHERE model=false` has to
be run and model is char). Because of the fact that this query never
runs and fails opportunity is lost.
task-3248489

closes odoo/odoo#123488

X-original-commit: 6c0d2d7a9d44459f3e09a38bd80ef9b018e8c946
Signed-off-by: Stéphane Debauche (std) <std@odoo.com>

addons/mail/models/mail_thread.py

@@ -1065,7 +1065,7 @@ class MailThread(models.AbstractModel):
                 is_a_reply = False
                 rcpt_tos_valid_localparts = [to for to in rcpt_tos_valid_localparts if to in other_model_aliases.mapped('alias_name')]
 
-        if is_a_reply:
+        if is_a_reply and reply_model:
             reply_model_id = self.env['ir.model']._get_id(reply_model)
             dest_aliases = self.env['mail.alias'].search([
                 ('alias_name', 'in', rcpt_tos_localparts),

addons/test_mail/tests/test_mail_gateway.py

@@ -1763,6 +1763,36 @@ class TestMailgateway(MailCommon):
         # This explains the multiple "�" in the attachment.
         self.assertIn("Chauss������e de Bruxelles", record.message_ids.attachment_ids.raw.decode())
 
+    def test_message_route_reply_model_none(self):
+        """
+        Test the message routing and reply functionality when the model is None.
+
+        This test case verifies the behavior of the message routing and reply process
+        when the 'model' field of a mail.message is set to None. It checks that the
+        message is correctly processed and associated with the appropriate record.
+        The code invokes function `format_and_process` to automatically test rounting
+        and then makes checks on created record.
+
+        """
+        message = self.env['mail.message'].create({
+            'body': '<p>test</p>',
+            'email_from': self.email_from,
+            'message_type': 'email',
+            'model': None,
+            'res_id': None,
+        })
+
+        self.env['mail.alias'].create({'alias_name': 'test', 'alias_model_id': self.env['ir.model']._get('mail.test.gateway').id})
+        record = self.format_and_process(
+            MAIL_TEMPLATE, self.email_from, 'test@test.com',
+            subject=message.message_id, extra=f'In-Reply-To:\r\n\t{message.message_id}\n',
+            model=None)
+
+        self.assertTrue(record)
+        self.assertEqual(record._name, 'mail.test.gateway')
+        self.assertEqual(record.message_ids.subject, message.message_id)
+        self.assertFalse(record.message_ids.parent_id)
+
 
 @tagged('mail_gateway')
 class TestMailThreadCC(MailCommon):