[FIX] mail: Reserve mis-referenced trackings to system users

Purpose
=======

Let's say that the field 'foo' is tracked and defined with a res.group.

When the field is modified, a mail.tracking.value is generated, but the
reference to the field name is a char field.

When displaying the mail.tracking.values on the chatter, a check is done
according to the field group to decide whether we should display it or not
to the user. See: odoo/odoo@c7aa8c5#diff-ad8b6db158187579d2208f233d993c3cR43

So if I rename the field, and if the mail.tracking.value is not modified,
the mail.tracking.value magically appears to the users who shouldn't
access it before.

Note: If the migration is correctly handled, this shouldn't be the case.
But manual manipulations on the database could lead to this issue.

Specification
=============

If the field referenced by the mail_tracking_value doesn't seem to exist,
then display its value to system users only, by security.

closes odoo/odoo#39033

Taskid: 2088634
X-original-commit: ae1e70eba10112170283cfc17fa95d94dd948d2b
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>

addons/mail/models/mail_tracking_value.py

@@ -40,7 +40,7 @@ class MailTracking(models.Model):
         for tracking in self:
             model = self.env[tracking.mail_message_id.model]
             field = model._fields.get(tracking.field)
-            tracking.field_groups = field.groups
+            tracking.field_groups = field.groups if field else 'base.group_system'
 
     @api.model
     def create_tracking_values(self, initial_value, new_value, col_name, col_info, tracking_sequence):