Set _regular_price 0 if empty in woocommerce product

The following will always set the price to default to 0 when creating/updating a product

function wpa104760_default_price( $post_id, $post ) {

    if ( isset( $_POST['_regular_price'] ) && trim( $_POST['_regular_price'] ) == '' ) {
        update_post_meta( $post_id, '_regular_price', '0' );
    }

}
add_action( 'woocommerce_process_product_meta', 'wpa104760_default_price' );

Again from the answer above you and the answer to your question, to always allow the product to be purchasable regardless of price existing:

 add_filter('woocommerce_is_purchasable', '__return_TRUE'); 

Both of these pieces of code can be added to your functions.php file.