Only update token meta after updating token default if the meta key is found

[james-allan]

Fixes https://github.com/woocommerce/woocommerce-subscriptions/issues/4631

Description

If you update your default payment method on the My Account → Payment methods screen and the Payment method doesn't integrate with the woocommerce_subscription_payment_meta filter (eg WooPayments), then additional meta will be added to the subscription without a key.

^{Notice the final row of the has an empty meta key}

This was caused by the foreach loop I've changed in this PR. If the loop didn't find a meta key used for tokens, it would just set it to an empty key because that's the default.

This PR fixes that.

How to test this PR

1. Install WooPayments
2. Install Woo Subscriptions
3. Create 2 subscription products with different billing frequencies (weekly and monthly).
4. Purchase the products together using a standard card 4242424242424242.
5. Make another purchase using a different card eg 4000056655665556.
6. Go to My Account → Payment methods
7. Change the 4000056655665556 payment method to your default.

8. In the notice that appears on the next page, click "Yes".
   • On trunk, check the order meta table for your subscription and you should notice there is an empty meta key with the token ID stored.
   • On this branch, make another subscription purchase using a new card, repeat steps 6-8 no empty additional meta key should be stored.

Zip of this branch for convenience: woocommerce-subscriptions-core.zip

trunk	This branch

Product impact

• [x] Added changelog entry (or does not apply)
• [ ] Will this PR affect WooCommerce Subscriptions? yes/no/tbc, add issue ref
• [ ] Will this PR affect WooCommerce Payments? yes/no/tbc, add issue ref
• [ ] Added deprecated functions, hooks or classes to the spreadsheet

[james-allan]

I've also just double checked that using a gateway like Stripe which does use a meta key, it still works to update the token stored on subscriptions.