[FIX] base: don't fail on empty groups attribute

In 15.0 this was supported. It may be also handy when editing views to
momentarily set the groups to `""`.

Steps to reproduce:
1. Install Odoo 15 locally
2. Edit or create a view with `groups=""` for some component
3. Upgrade to 16.
It fails.

Empty groups was allowed in 15.0 we want to ensure this is not broken
unintentionally anymore, such a new test was added.

Muted logged to hide the warning (also present in 15.0):
```
2023-02-08 11:09:18,697 506777 WARNING test_16_gr odoo.addons.base.models.ir_ui_view: The group '' defined in view does not exist!
View error context:
{'file': None,
 'line': 3,
 'name': 'foo',
 'view': ir.ui.view(242,),
 'view.model': 'res.partner',
 'view.parent': ir.ui.view(),
 'xmlid': ''}
 ```

closes odoo/odoo#112062

Signed-off-by: Denis Ledoux (dle) <dle@odoo.com>