In #1923 we changed reg.Contact to a pointer, which can be nil if the corresponding data from the DB is the literal string "null". This causes panics in expiration-mailer, which we need to fix.
This change fixes modelToRegistration to always return a pointer to a non-nil slice. It also adds an extra sanity check in expiration-mailer itself.
Fixes#1993https://github.com/letsencrypt/boulder/pull/1997
Kleber Correia