How to avoid saving empty data to sql while using add_meta_box

I'm trying not to save empty data (null) while using add_meta_box function.
My code is

/**
 * Register meta boxes.
 */
function hcf_register_meta_boxes() {
    add_meta_box( 'hcf-1', __( 'Hello Custom Field', 'hcf' ), 'hcf_display_callback', 'post' );
}
add_action( 'add_meta_boxes', 'hcf_register_meta_boxes' );

/**
 * Meta box display callback.
 *
 * @param WP_Post $post Current post object.
 */
function hcf_display_callback( $post ) {
    include plugin_dir_path( __FILE__ ) . './form.php';
}

/**
 * Save meta box content.
 *
 * @param int $post_id Post ID
 */
function hcf_save_meta_box( $post_id ) {
    if ( defined( 'DOING_AUTOSAVE' )  DOING_AUTOSAVE ) return;
    if ( $parent_id = wp_is_post_revision( $post_id ) ) {
        $post_id = $parent_id;
    }
    $fields = [
        'hcf_author',
        'hcf_published_date',
        'hcf_price',
    ];
    foreach ( $fields as $field ) {
        if ( array_key_exists( $field, $_POST ) ) {
            update_post_meta( $post_id, $field, sanitize_text_field( $_POST[$field] ) );
        }
     }
}
add_action( 'save_post', 'hcf_save_meta_box' );

I'm using code from https://medium.com/meta-box/how-to-create-custom-meta-boxes-custom-fields-in-wordpress-4703e53638bd .

However whenever I save my post, the null data always be stored to wp_postmeta(sql) eventhough I didn't put data in the metabox and it is making a lot of garbage rows to my sql. I don't want to save null data to my sql. Please help me with this. Thanks.