Fix SoapClient serialization error on checkout AJAX requests.

Kavin Grequested to merge
issue/commerce_usaepay-3587054:3587054-soapclient-stored-as into 2.0.x

Fix SoapClient serialization error on checkout AJAX requests.

Also fix implicit nullable parameter deprecations for PHP 8.4.

Fixes: #3587054

Closes #3587054

Edited by Kavin G

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