fix(resources): Make tax_rates default to empty array everywhere

[bravier]

Before, tax_rates property could either be null or an empty array at different places in the code despite previous commits 84fa00b and 4f7c8e3.

Commit 4f7c8e3 even broke localstripe tests.

Stripe docs says InvoiceItems' and SubscriptionItems' tax_rates is an array:

• https://stripe.com/docs/api/invoiceitems/object#invoiceitem_object-tax_rates
• https://stripe.com/docs/api/subscription_items/object#subscription_item_object-tax_rates

So let's use arrays everywhere.

Note: same commit could be done for default_tax_rates.