ACF meta_query Pre-Fix

gistfile1.php

<?php
	$today = date('Ymd');
	$meta_query = array(
		'relation' => 'OR',
		array(
			'key' => 'custom_date',
			'compare' => '>=',
			'value' => $today
		),
		array(
			'key' => 'custom_date',
			'compare' => 'NOT EXISTS',
			'value' => ''
		)
	);
	$query->set( 'meta_query', $meta_query );