Contacts preferred_mail_format can't be NULL

See http://civicrm.stackexchange.com/a/15965/104.

I'm not enterely sure but I think that due last solved issues (CRM-18157, CRM-19135, CRM-19442, CRM-19423) the following sql needs to be added to the upgrade script one more time.

UPDATE `civicrm_contact` SET `preferred_mail_format` = 'Both' WHERE `preferred_mail_format` IS NULL;