[FIX] core: don't fail when reading recordset with ids [None]

The revision 65a012c2 changed the
behavior of method _read() when invoked on a record with id None.
Before the change, the method would consider that no records have been
fetched because the id is falsy.  After the change, it keeps the id None
in the fetched records, and attempts to make subsequent SQL queries like
"column IN ()", which is syntactically incorrect.

OPW 3104957

closes odoo/odoo#108771

Signed-off-by: Rémy Voet <ryv@odoo.com>