NoMethodError : `undefined method fail_open?' for nil:NilClass from Security::UnenforceablePolicyRulesNotificationWorker

There's a race condition that can trigger when e.g. a security policy project is unassigned. It doesn't affect the functionality of fallback_behavior, but it will eat into our error budget.

Sentry: https://new-sentry.gitlab.net/organizations/gitlab/issues/776115/?project=3&query=fail_open&referrer=issue-stream&statsPeriod=14d&stream_index=0