Auto Generate Title for Custom Post – Stop Overwright when updating post

Hi,

I am using this snippet in my child theme functions.php to automatically generate a title (which I use as an identifier for an inventory) for my custom posts resulting in the prefix p’ and the ‘date’: eg Name: p-2014061184244 or Title: P-2014061184244.

Problem is that every time I update my post it overwrites my previous post title. How can I stop it to overwrite if my title already has a value? eg if post already has title leave it – if post has no title generate title.

<?php
    add_filter('wp_insert_post_data','dsgnwrks_update_title',99,2);
    function dsgnwrks_update_title($data, $postarr) {
    global $post;
    if ( !is_admin() )
    return $data;

    if ($data['post_type'] == 'my-custom-post-type') {

    $data['post_name'] = sanitize_title( 'p'.date_i18n( 'YmjGis', strtotime( $post->post_date ) ) );

    $data['post_title'] = 'P'.date_i18n( 'YmjGis', strtotime( $post->post_date ) );

    return $data;
    } else {
    return $data;
    }
    }

?>

Thank you for your help ??