Order/order item price could be NULL

A similar problem arises in the module file, if the price is null:

TypeError: Drupal\commerce_price\Rounder::round(): Argument #1 ($price) must be of type Drupal\commerce_price\Price, null given, called in /var/www/html/web/modules/custom/facebook_pixel/modules/facebook_pixel_commerce/facebook_pixel_commerce.module on line 61 in Drupal\commerce_price\Rounder->round() (line 29 of modules/contrib/commerce/modules/price/src/Rounder.php).

Let's fix that here as well.

Cleaned up the existing code a bit and fixed the module file implementation. Please review!

Note, that by the official meta documentation, "value" doesn't seem to support NULL values, so let's use "0" where we can.

Nice, good enough to fix the issue. Can't be perfect, I think...

Thanks all!