Fix fatal error when editing the shortcode checkout page

[wjrosa]

Fixes #3757

Changes proposed in this Pull Request:

This PR fixes a fatal error thrown when editing the shortcode checkout page on PHP 8.4 with an empty cart:

PHP Fatal error: Uncaught Error: Call to a member function needs_shipping() on null in */wp-content/plugins/woocommerce-gateway-stripe/includes/payment-methods/class-wc-stripe-express-checkout-helper.php:629

The fix consists on checking the existence of the cart object before calling the needs_shipping method.

Testing instructions

Code review should be enough. If you want to test it:

• Checkout and build this branch on your test environment (fix/fatal-error-editing-shortcode-checkout)
• Make sure you are running your store on PHP 8.4
• As a merchant, edit your shortcode checkout page (create one before if you don't have it)
• Confirm no fatal errors are thrown

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)