[FIX] core: o2m modifiers in the SSF

Because the sub-record values would not necessarily get fetched
ever (whether default or stored), the computation of modifiers might
blow up if it relied on one of the un-fetched un-specified fields.

One such situation is trying to create a partner with child partners
if base_address_city is installed: the module adds a readonly attr
predicated upon the parent_id, without explicitly providing such the
field would be missing from the O2M record's values.

Closes #37176

closes odoo/odoo#37408

Signed-off-by: Christophe Simonis <chs@odoo.com>