display loop only if a post meta data exist

Question

If you want to avoid preprocessing the data, this will work (though it’s definitely the quick and dirty method):

<?php while (have_posts()) : the_post(); ?>
    <?php $data = get_post_meta( $post->ID, 'key', true ); ?>
    <?php if( $data != '' ) : ?>
        <!-- info --> 
    <?php endif; ?>
<?php endwhile; ?>

Otherwise (and this is definitely the better way to go if you need the best functionality), you can do a preliminary loop through the data (or even get different data using the meta), determine if any of the posts have meta, then conditionally execute the loop.