nastedQuery

$meta_query = array(
	'relation' => 'OR',
	array(
		'key'     => 'next_airing_episode_date',
		'value'   => 'time()',
		'compare' => '<=',
		'type'    => 'NUMERIC',
	),
	array(
		'key'     => 'next_airing_episode_date',
		'compare' => 'NOT EXISTS',
	),
);