Drupal 8 Contextual Filter IS NULL via hook_views_query_alter()

MYMODULE.views_execution.inc

<?php

use Drupal\views\ViewExecutable;
use Drupal\views\Plugin\views\query\QueryPluginBase;

/**
 * Implements hook_views_query_alter().
 *
 * @param ViewExecutable $view
 * @param QueryPluginBase $query
 */
function MYMODULE_views_query_alter(ViewExecutable $view, QueryPluginBase $query) {
  // apply this to all group event views
  if (strpos($view->id(),'group_event') !== false) {
  
    cp_event_views_contextual_is_null($query);
    
  }
}

/**
 * Convert queries using our special 'null' value in contextual filters into 'IS NULL' clauses
 *
 * How to use:
 *  - Add a contextual filter
 *  - Set a default value of 'null'
 *
 * @param QueryPluginBase $query
 */
function cp_event_views_contextual_is_null(QueryPluginBase &$query) {

  // loop over where groups
  if (is_array($query->where)) {
    foreach ($query->where as $whereIndex => $where) {

      // loop over where group conditions
      if (is_array($where['conditions'])) {
        foreach ($where['conditions'] as $conditionIndex => $condition) {

          // loop over group condition values
          if (is_array($condition['value'])) {
            foreach ($condition['value'] as $valueIndex => $value) {

              // if the value is the string of 'null'
              if ($value === 'null') {

                // convert the condition to a 'IS NULL' query
                $query->where[$whereIndex]['conditions'][$conditionIndex]['field'] = strstr($condition['field'], ' =', true);
                $query->where[$whereIndex]['conditions'][$conditionIndex]['value'] = NULL;
                $query->where[$whereIndex]['conditions'][$conditionIndex]['operator'] = 'IS NULL';
              }
            }
          }
        }
      }
    }
  }
}

/**
 * Source:
 * https://www.chapterthree.com/blog/using-hook-views-query-alter
 */