preferred_mail_format should never be NULL or handled properly

if "nothing" is selected on a contact's preferred e-mail format, then empty mails are being sent.

Either "nothing" shouldn't be an allowed option or it should be handled by CiviMail as "both".
Looking into the database I see that there are both empty values and NULL in that column. Empty values are currently treated as "both". I think it should be the same for null. Sending an empty e-mail is bad, especially as the user doesn't see it.