[FIX] mail: this.env.services.bus_service is undefined

Before this commit, a page reload or a redirect could raise following
error:

```
Uncaught (in promise) TypeError: this.env.services.bus_service is undefined
```

This happens due to `Messaging` model relying on `bus_service` in its
teardown method `_willDelete`. This is sometimes unsafe because the
bus service may not have been deployed yet.

Task-2468469

closes odoo/odoo#66818

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