[FIX] base: traceback on rpc call if data contain default dict

before this commit, on reading data with default dict data
type is showing error to end user, without returning the
requested data.

for eg, if a read operation is triggered on model sale.order
it wont return the requested data, instead traceback is
shown in response.

in sale.order model the tax_totals field is a computed
field, with data as format default dict which was
causing the issue.

after this commit, without any traceback the requested
data will be returned to the user.

closes odoo/odoo#129830

Signed-off-by: Julien Castiaux (juc) <juc@odoo.com>