[FIX] digest: fix digest mail failure due to blank "From" field

Currently while sending a digest, if a company does not have an email
address set, the "From" field for the mail is blank and so trying to
send digest fails.

With this commit, if a company does not have an email address set, it
will fallback to logged in user's email address as a "From" value.
In most cases, it will be the Odoobot email address (if the digest is
sent with the CRON) but if the admin send manually the digest, we will
use the admin email address.

TaskID-2729780

closes odoo/odoo#86507

X-original-commit: b9f8b8c7
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>