[FIX] base: skip unneeded update_list call

`update_list` is an expensive operation and could be avoided when no modules are
updated. Particually, it speeds up initial theme installation on website, which
has following code:

```
    themes.filtered(lambda m: m.state == 'installed').button_upgrade()
```

Benchmark for Odoo 15: `/website/configurator_apply` spends 1.3 sec for unneeded
`update_list` call.

closes odoo/odoo#74935

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>