[FIX] mail: new messages extra domain

A default MAX(id) was added in the extraDomain when there are no messages
to create it. This validation already existed but was omitted in the
refactoring - https://github.com/odoo/odoo/pull/57061

Current behavior before PR:

If there are no messageIds, the method loadNewMessages returns id > null
and this domain returns 0 records

https://github.com/odoo/odoo/blob/7d992af9561c499ef1912453f3b697bad18e441f/addons/mail/static/src/models/thread_cache/thread_cache.js#L55



Desired behavior after PR is merged:

Create the extraDomain with a default MAX(id) when there are no messages
to create it and continue with the normal flow of loadMessages

closes odoo/odoo#91610

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>