How to display thumbnail if post is assigned one otherwise not

you just need to clean up the if statement:

<?php if ( has_post_thumbnail() ) { ?>
<div class="col-md-3">
    <?php the_post_thumbnail(); ?>
</div>
<?php } ?> 

here’s the whole bit cleaned up:

 <?php query_posts('posts_per_page=1') ?>
            <?php while ( have_posts() ) : the_post(); ?>

                <div class="jumbotron">
                    <?php if ( has_post_thumbnail(); { )?>
                    <div class="col-md-3">
                        <?php the_post_thumbnail(); ?>
                    </div>
                    <?php } ?>
                    <div class="col-md-9">
                        <h2><a href="https://wordpress.stackexchange.com/questions/129375/<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>
                        <p><?php the_excerpt(); ?></p>
                        <p class="text-muted">Posted on <?php the_time('jS F, Y'); ?></p>
                    </div>

                </div>

            <?php endwhile; wp_reset_query(); ?>