[FIX] l10n_it_edi: fix traceback on fetchmail

When the server execute the job 'Mail: Fetchmail Service'
if it's processed an attachment having a Notes field with None values
the following traceback will occur
AttributeError: 'NoneType' object has no attribute 'split'

//IdentificativoSdI','//DataOraRicezione','//Descrizione','//MessageId','//Note'.
seems to be not mandatory

opw-2277722

closes odoo/odoo#54591

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>