Issue #3443566 missing case where the signup form is being submitted by...

Issue #3443566 missing case where the signup form is being submitted by someone who is signed up in Mailchimp but selects no interests in Drupal. They should not be unsubscribed.

This is already being attempted by mailchimp_lists_process_subscribe_form_choices but unfortunately by the time we're calling postSave on the field, isNew() returns false, because the entity has an ID at that point. Because mailchimp_lists_process_subscribe_form_choices is called by a few things, I'm not comfortable changing the logic there.

I also ran into a bug where mailchimp_interest_groups_form_elements was erroring out because the value of $groups_default was "FALSE".