[FIX] core: find currency when writing monetary field

When creating a record, if a value is related to a monetary field and if
the currency field is a non-stored related one, the value will not be
rounding: when writing the monetary value in the database, we first call
`convert_to_column`. In the parameter, `record` is empty (it is not yet
created) and `values` only contains the stored values (so the currency
value is not present). As a result, `currency` will not be defined and
the value will not be rounded.

OPW-2955202

closes odoo/odoo#104346

Signed-off-by: Raphael Collet <rco@odoo.com>