[FIX] mail: allow message_log to store a void email_from

Some functional flows use sudo() to create records as current user is the
public user. This user has no email. Since 1e6c3bec using sudo does
not take superuser identity completely as it only bypasses access rights.
It means some messages are created with public user as author, meaning no
email_from.

In this commit we fix some functional flows by allowing message_log to store
messages with a void email_from. Indeed message_log does not propagate any
notifications, meaning email_from is not required in any email notification
or to compute a reply_to.