If `default email digest frequency` was set to "Never", users would get
a `digest_after_minutes` set to `nil` which triggered this error
in the logs if/when the site eventually changed that setting and
enabled digests:
```
NoMethodError (undefined method `>=' for nil:NilClass)
/var/www/discourse/app/mailers/user_notifications.rb:227:in `digest'
```
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