When a portal user is created (e.g. through the `auth_signup` module),
an unnecessary write is done on the `write_date` of the default digest.
This `write` is unnecessary since a portal user is never subscribed to
the default digest.
In case of a high signup frequency, it can cause concurrent transaction
errors.
We avoid writing if no internal user is being created.
closesodoo/odoo#162563
X-original-commit: bbc427e837b08187f351febfde4339262e660944
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>