[FIX] mail: avoid crash for hidden chat windows without channel

Steps to reproduce
==================

- Install Time Off
- With Mitchell Admin, set notifications to "Handle in Odoo"
- With Marc Demo, create a few Holidays requests
- Switch back to Mitchell Admin
- Open enough chat windows from the menu in the navbar so that it fills
  the entire window

When there is no place left, we get this error

```
Cannot read properties of undefined (reading 'localMessageUnreadCounter')
```

Cause of the issue
==================

Threads related to time off have no channel

opw-3204260

closes odoo#120418

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

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

@@ -239,7 +239,7 @@ registerModel({
                 );
                 let amount = 0;
                 for (const chatWindow of allHiddenWithThread) {
-                    if (chatWindow.thread.channel.localMessageUnreadCounter > 0) {
+                    if (chatWindow.thread.channel && chatWindow.thread.channel.localMessageUnreadCounter > 0) {
                         amount++;
                     }
                 }