[13.x] Adding metered product to existing subscription fails due to quantity key being present

[hailwood]

• Cashier Version: 13.0.0
• Laravel Version: 8.46.0
• PHP Version: 8.0.3
• Database Driver & Version: Mysql 8

Description:

Seems like stripe doesn't like us sending across quantity as null. It wants either an integer or the key missing entirely.

If we filter this array to remove null values the subscription is updated as intended.

If we do not then we get a Stripe exception Stripe\Exception\InvalidRequestException with the message

Invalid integer:

Stacktrace

/app/vendor/stripe/stripe-php/lib/Exception/InvalidRequestException.php:35
--
/app/vendor/stripe/stripe-php/lib/ApiRequestor.php:189
/app/vendor/stripe/stripe-php/lib/ApiRequestor.php:151
/app/vendor/stripe/stripe-php/lib/ApiRequestor.php:489
/app/vendor/stripe/stripe-php/lib/ApiRequestor.php:120
/app/vendor/stripe/stripe-php/lib/BaseStripeClient.php:134
/app/vendor/stripe/stripe-php/lib/Service/AbstractService.php:59
/app/vendor/stripe/stripe-php/lib/Service/SubscriptionItemService.php:62
/app/vendor/laravel/cashier/src/Subscription.php:834
/app/vendor/laravel/cashier/src/Subscription.php:890

This is not an issue when creating a new subscription as we only add the quantity key if the value is not null.

Steps To Reproduce:

1. Create a subscription with an existing product
2. Attempt to update the subscription to add an additional metered product

$billable->subscription()->addMeteredPrice('price_ab123');

[driesvints]

Urgh, should have added a test for that. Thanks @hailwood.