Fix call to get delivery_method

It's supposed to default to PRINT but ends up defaulting to NULL if missing. However the only way this would ever come up is if you installed the extension but then instead of filling out the settings page in the UI you set the settings with a script and left out delivery_method, but even then it would just fall through to the code below it which defaults to PRINT and either chooses PRINT or EMAIL. So you'd never notice.