Fixed undefined quantity notice

[Lyrisbee]

Fixed Stripe Notice: Undefined property of Stripe\SubscriptionItem instance: quantity when creating or swapping to a single metered price.

Test:

cashier-stripe % ./vendor/bin/phpunit --filter test_swap_metered_price_to_different_price
PHPUnit 9.5.20

Stripe Notice: Undefined property of Stripe\SubscriptionItem instance: quantity
Stripe Notice: Undefined property of Stripe\SubscriptionItem instance: quantity
.Stripe Notice: Undefined property of Stripe\SubscriptionItem instance: quantity
Stripe Notice: Undefined property of Stripe\SubscriptionItem instance: quantity
.                                                                  2 / 2 (100%)

Time: 00:23.421, Memory: 28.00 MB

OK (2 tests, 24 assertions)

[github-actions]

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