[FIX] base_automation: do not read all fields

vals may be an empty dict
This may happen a write is done on a computed field, the call to _write will
be empty (still needed to update write_uid/date).

Before this commit, all fields were read in the read call.
This was unecessary and may produce acess-rights errors or other side effect
(e.g. in odoo/enterprise#3778 the technical field ticket_count was read, even
if not present in the view or the write call)

Only compute old_values on the fields that are modified

Fixes odoo/enterprise#3778
opw-1949911

closes odoo/odoo#31737

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