start-date-not-exists

$additional_meta_query = array(
	'relation' => 'AND',
	array(
		'relation' => 'AND',
		array(
			'key'     => '__start_date',
			'compare' => 'NOT EXISTS',
		),
	),
	array(
		'key'     => '_end_date',
		'compare' => 'NOT EXISTS',
	),
);