Describe the bug

The following was spotted occurring regularly on Atomic sites.

PHP Warning:  Undefined array key "required" in /srv/htdocs/wp-content/plugins/woocommerce-payments/includes/class-wc-payments-checkout.php on line 249

This warning appears to occurs on this line, introduced by #9707, and will produce a warning when a WC checkout billing field does not have a required property.

This warning appears to have been occuring live since the 8.6.0 release of the plugin and has been encountered in related WPORG forum posts, such as this one.

To Reproduce

The forum post above mentions that the error occurs in the shop and on product pages, but it should be triggerable from the checkout as well, when any billing field validations lack a required property. One way to encounter such a field is to ensure that a store's location is such that billing address fields (e.g. city, state, or postcode, etc.) are lacking a required property (see source here for reference) or one could simply add a custom field without any required validation.

Actual behavior

Code throws and logs a warning.

Screenshots

Atomic log

Expected behavior

Code should not throw any warnings.