When a non-taxable item is in the cart first, added taxable items are not taxed.

[darcie]

Related to #2514 likely.

Steps to Reproduce:

1. Create a product that is not taxable and a product that is taxable
2. Add the non-taxable item to the cart
3. Add the taxable item to the cart
4. Use an address that falls within the tax nexus
5. The total tax is zero
6. An error is logged Error retrieving the tax rates. Received (406): {"status":"406","error":"Not Acceptable","detail":"line_items is invalid"} (WCS Tax)
7. A rate is added to Standard Rates for the 0.00% tax rate for the postal code used.

I can reproduce this in WooCommerce Shipping & Tax 1.25.23 and 1.25.22, it does not happen in 1.25.21. Due to this, I believe it is related to Pull #2516.

Screenshots:

Logs:

When no rate is returned:

03-16-2022 @ 09:43:00 - :::: TaxJar Plugin requested :::: (WCS Tax)
03-16-2022 @ 09:43:00 - :::: TaxJar API called :::: (WCS Tax)
03-16-2022 @ 09:43:00 - Requesting: taxjar/v2/taxes - {"from_country":"US","from_state":"CA","from_zip":"90210","from_city":"Beverly Hills","from_street":"123 Woo Way","to_country":"US","to_state":"CA","to_zip":"90210","to_city":false,"to_street":false,"shipping":"5","plugin":"woo","line_items":{"1":{"id":"41-3416a75f4cea9109507cacd8e2f2aefc","quantity":1,"product_tax_code":"","unit_price":"18","discount":"0"}}} (WCS Tax)
03-16-2022 @ 09:43:01 - Error retrieving the tax rates. Received (406): {"status":"406","error":"Not Acceptable","detail":"line_items is invalid"} (WCS Tax)
03-16-2022 @ 09:43:01 - Received: none. (WCS Tax)

Line items when the rate is successfully returned:

"line_items":[{"id":"41-3416a75f4cea9109507cacd8e2f2aefc","quantity":1,"product_tax_code":"","unit_price":"18","discount":"0"}]}

Reported in 4857330-zen and 4803765-zen

[hmadisonturner]

This is due to the array_filter() call in #2516 returning a sparse array, which is then encoded as an object by wp_json_encode( $body ). Wrapping the array_filter() call in array_values() fixes it, but it feels gross.

[darcie]

Another example in 4863532-zen

[nawaz0705]

Reported in 4876335 - Zen