Add ‘if exists’ to filter

You want has_post_thumbnail.

add_filter( 'the_content', insert_featured_image, 20 );
function insert_featured_image( $content ) {
  global $post;
  if (has_post_thumbnail($post->ID)) {
    $content = preg_replace( "/<\/p>/", "</p>" . get_the_post_thumbnail($post->ID, 'post-single'), $content, 1 );
  }
  return $content;
}

Note: The code above is from the post referenced in the OP, but with the has_post_thumbnail conditional. I also had to add global $post; to avoid the “undefined variable” Noticess. Alternately, you can leave that $post references out as has_post_thumbnail and get_the_post_thumbnail will assume the global $post (which is why the original code works at all).

add_filter( 'the_content', insert_featured_image, 20 );
function insert_featured_image( $content ) {
  if (has_post_thumbnail()) {
    $content = preg_replace( "/<\/p>/", "</p>" . get_the_post_thumbnail(null, 'post-single'), $content, 1 );
  }
  return $content;
}