If option: show this. Else show nothing

Try this:

<?php
$options = get_option( 'schema_theme_options' );
$meta_desc = $options['metadescription'];
?>
<?php if( ( is_home() || is_front_page() ) && '' !== $meta_desc ) : ?>
    <meta name="description" content="<?php echo $meta_desc; ?>">
<?php endif; ?>

It’s a bit neater and a bit more foolproof than the other proposed solution.