[FIX] mail: reading property from undefined

To Reproduce
============
- add event on calendar
- edit the event and remove the user from participants list and add another one.
- save the record -> traceback

Problem
=======
we try to read `hasReadAccess` from `this.followedThread` that is `null` in this case.

Solution
========
check if `this.followedThread` is `null` or not

opw-3126545

closes odoo/odoo#109721

X-original-commit: f9a23af63bb5fe882f0d42f06ef1406910836115
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/mail/static/src/models/follower.js

@@ -169,8 +169,11 @@ Model({
                 const hasWriteAccess = this.followedThread
                     ? this.followedThread.hasWriteAccess
                     : false;
-                return this.messaging.currentPartner === this.partner
+                const hasReadAccess = this.followedThread
                     ? this.followedThread.hasReadAccess
+                    : false;
+                return this.messaging.currentPartner === this.partner
+                    ? hasReadAccess
                     : hasWriteAccess;
             },
         }),