how to post default thumbnail if post not created yet

by

Create “images” folder in current active theme the put “https://wordpress.stackexchange.com/questions/332635/dummy-image-1-1.jpg” in that folder.

<?php  
if ($the_query->have_posts() ) :
    while ($the_query->have_posts()) : $the_query->the_post(); 
        if ( has_post_thumbnail( ) ) {
            the_post_thumbnail( 'full', array( 'class' => 'mySlides' ) ); 
        } else {
        ?>
            <img src="https://wordpress.stackexchange.com/questions/332635/<?php echo get_template_directory_uri()."/images/dummy-image-1-1.jpg'; ?>" class="mySlides" height="350px"/>
        <?php
        }
    endwhile;
else:
?>
    <img src="https://wordpress.stackexchange.com/questions/332635/<?php echo get_template_directory_uri()."/images/dummy-image-1-1.jpg'; ?>" class="mySlides" height="350px"/>
<?php
endif;
wp_reset_postdata(); 
?>

Use path according to the theme(parent or child)