Details
Description
Steps to reproduce on dmaster:
1. Switch language with tranlations for preferred_mail_format, e.g. dutch
2.1 Open a contact and inline-edit communication preferences
2.2 Edit a contact and go to communication preferences
3. preferred_mail_format is not selected for translated labels resulting in stroring a NULL value (it works fine for untranslated value 'HTML')
Changing the preferred_mail_format field to be required at least prevents storing unintentional NULL values as proposed here: https://github.com/civicrm/civicrm-core/pull/8771