Refrain from setting email opt-in preference when checkbox is missing

As per [ECOM-717](https://openedx.atlassian.net/browse/ECOM-717). If the email opt-in feature is enabled, but the checkbox is missing (either because Drupal isn't passing an organization name to the courseware views, or because of an A/B test removing it from the DOM), no opt-in preference is saved. This allows default logic in the management command to take over and mark users with no email opt-in preference(s) as opted-in.

@stephensanchez and @wedaly, could you review this?
You may be able to simplify this whole if/else statement by doing
href_url = 'accounts_login' if settings.FEATURES.get("ENABLE_COMBINED_LOGIN_REGISTRATION") else 'register_user'
window.top.location.href = $("a.register").attr("href") || "${reverse(href_url)}?course_id=${course.id | u}&enrollment_action=enroll" + emailOptIn;