Remove pagination if search returns empty

add this code in your search.php

<?php 
$show_pagination = true;
if($wp_query->post_count < 0) {
    $show_pagination = false;
}
?>

<?php if($show_pagination){ ?>
    <!-- The pagination component -->
            <div class="container">
                <div class="row mt-3">
                    <?php understrap_pagination(); ?>
                </div>
            </div>
<?php } ?>

So your search.php look like this :

<div class="wrapper" id="search-wrapper">
<div class="<?php echo esc_attr( $container ); ?>" id="content" tabindex="-1">
    <div class="row">
        <main class="container" id="main">
            <?php if (have_posts() && strlen( trim(get_search_query()) ) != 0 ) : while (have_posts()) : the_post(); ?>
            <header class="page-header">
                <div class="container">
                    <div class="row">
                        <div class="col-sm-12 text-center">
                            <div class="">
                                <h1>Zoekresultaten voor: 
                                    <?php   
                                    printf(
                                        /* translators: %s: query term */
                                        esc_html__( ' %s', 'understrap' ),
                                        '<span>' . get_search_query() . '</span>'
                                    );
                                    ?>
                                </h1>
                            </div>
                        </div>
                    </div>
            </header>
            <div>
                <h4>
                    <?php /* Start the Loop */ ?>
                    <?php get_template_part( 'loop-templates/content', 'search' ); ?>
                </h4>
            </div>
            <?php endwhile; else:?>
                <div class="container whitespace">
                    <div class="row">
                        <div class="col-lg-12 text-center">
                            <p class="display-4">Geen Resultaten..</p>
                        </div>
                    </div>
                </div>
            <?php endif; ?>
        </main>

        <?php 
       $show_pagination = true;
       if($wp_query->post_count < 0) {
            $show_pagination = false;
       }
      ?>
     <?php if($show_pagination){ ?>
       <!-- The pagination component -->
        <div class="container">
            <div class="row mt-3">
                <?php understrap_pagination(); ?>
            </div>
        </div>
      <?php } ?>
    </div>
</div>

let me know if it helps you!