Want to make A CSS if meta key value is empty

you won’t need css for that

// Create the widget output.
public function widget( $args, $instance ) {
$prod_image_url = get_post_meta( 'prod_image_url' );
if(!empty($prod_image_url)){
echo $args['before_widget'] . $args['before_title'] . $args['after_title']; 

if( is_single() ):
    ?>
    <div id="show">
        <img class="prodimage" src="https://wordpress.stackexchange.com/questions/253903/<?php echo ( get_post_meta( get_the_ID(),"prod_image_url', true ) ); ?>"/><br>
    </div>
    <?php
else:
    echo '';
endif;
}
?>

For any professional help you can consider checking out : PSD to WordPress