Filters out any empty merge_vars sent to MailChimp

tw_eea_mailchimp_filter_empty_merge_vars.php

<?php //Please do not include the opening PHP tag if you already have one

function tw_eea_mailchimp_filter_empty_merge_vars( $subscribe_args, $registration, $EVT_ID ) {
    //Filter null values from merge_vars.
    $subscribe_args['merge_fields'] = array_filter($subscribe_args['merge_vars'], function($merge_var) { return !is_null($merge_var); });
    return $subscribe_args;
}
add_filter('FHEE__EE_MCI_Controller__mci_submit_to_mailchimp__subscribe_args', 'tw_eea_mailchimp_filter_empty_merge_vars', 999, 3);