[FIX] board: empty dashboard fails to render on mobile

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

- Go on a 14.0 runbot
- Open dashboard

```
TypeError: Cannot read properties of undefined (reading 'split')
    at Class.changeLayout
```

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

`.oe_dashboard` isn't rendered on an empty dashboard

opw-3469183

closes odoo/odoo#132254

Signed-off-by: Romain Estievenart (res) <res@odoo.com>