Remove null filters from migrated D8 text formats

remove_null_filters.php

<?php

use Drupal\filter\Plugin\Filter\FilterNull;

/**
 * Run on migrated text formats which have "missing" or null filters.
 *
 * @see \Drupal\filter\FilterFormatFormBase::form()
 */
function _update_text_formats() {
  foreach (filter_formats() as $format) {
    $filters = $format->filters();
    foreach ($filters as $filter_id => $filter) {
      if ($filter instanceof FilterNull) {
        $filters->removeInstanceID($filter_id);
      }
    }
    $format->save();
  }
}