PHP - If variable is not empty echo some html code

Question

If a variable is not empty, I would like to display some HTML code; otherwise, I would like to display nothing.

I attempted this code, but it is ineffective:

<?php 
    $web = the_field('website');
    if (isset($web)) {
?>
       <span class="field-label">Website: </span><a href="http://<?php the_field('website'); ?>" target="_blank"><?php the_field('website'); ?></a> 
<?php
    } else { 
        echo "Niente";
    }
?>