[FIX] web: basic_model: save a paged nested o2m

In a form view, have a o2m field (A), with a nested o2m field (B).
The A field should have multiple pages, with each record having at least one record
in their B field.

Have a (C) field that will modify a record in the nested o2m when changed.

Change the C field, and save.

Before this commit, the record's nested o2m from the other page had a wrong value
(a mere 4 command).

After this commit, their value is a command 4 and a command 1, with the right values.

closes odoo/odoo#78707

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>