Bug 30301 added the option to force mandatory account expiration notices for a given patron category in the membership_expiry cron. In these cases, the primary contact method on the patron account is used to determine the transport type for the notice. However, primary contact method may not be a mandatory field depending on the system preference settings. If a patron doesn't have a primary contact method set for their account, this causes the notice not to get sent at all, even if the patron has one or more valid email addresses or SMS number on their account. We need a fallback to ensure the mandatory notice is sent in this case. I think it would make the most sense to specify transport type(s) in the patron category settings when making account expiration notices mandatory for that category.