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?
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