meta-relation-or-query.php

<?php
function demo_shortcode(){
$args = array(
	'post_type' => 'product',
	'meta_query'=> array(
		'relation' => 'OR',
		array(
			'key' => 'some_key',
			'value' => '42',
			'compare' => 'IN',
		),
		array(
			'key' => 'some_key',
			'compare' => 'NOT EXISTS',
		),
	),
);

$new_posts = WP_Query( $args );

// the loop

if ( $new_posts->have_posts() ) : while ( $new_posts->have_posts() ) : $new_posts->the_post(); ?>

    <!-- post content goes here -->
    <?php the_title(); ?>

<?php endwhile; else: ?>

    <h3>Oops!! Looks like something went wrong. Please get in touch with the site <a href="mailto:<?php echo get_bloginfo('admin_email'); ?>">administrator</a> and we'll get this sorted out</h3>

<?php endif; ?>

<?php wp_reset_postdata();
}
add_shortcode( 'demo_code', 'demo_shortcode' );