Fix fatal error with PRB buttons

[wjrosa]

Changes proposed in this Pull Request:

This PR fixes a possible fatal error happening when a product cannot be found, related to PRBs' add to cart ajax endpoint. This error was caught with Grafana:

PHP Fatal error: Uncaught Error: Call to a member function get_type() on bool in /wordpress/plugins/woocommerce-gateway-stripe/9.2.0/includes/payment-methods/class-wc-stripe-payment-request.php:1475 Stack trace: #0 /wordpress/core/6.7.2/wp-includes/class-wp-hook.php(324): WC_Stripe_Payment_Request->ajax_add_to_cart('') #1 /wordpress/core/6.7.2/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters('', Array) #2 /wordpress/core/6.7.2/wp-includes/plugin.php(517): WP_Hook->do_action(Array) #3 /wordpress/plugins/woocommerce/9.7.1/includes/class-wc-ajax.php(98): do_action('wc_ajax_wc_stri...') #4 /wordpress/core/6.7.2/wp-includes/class-wp-hook.php(324): WC_AJAX::do_wc_ajax('') #5 /wordpress/core/6.7.2/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters(false, Array) #6 /wordpress/core/6.7.2/wp-includes/plugin.php(517): WP_Hook->do_action(Array) #7 /wordpress/core/6.7.2/wp-includes/template-loader.php(13): do_action('template_redire...') #8 /wordpress/core/6.7.2/wp-blog-header.php(19): require_once('/wordpress/core...') #9 /wordpress/core/6.7.2/index.php(17): require('/wordpress/core...') #10 {main} thrown in /wordpress/plugins/woocommerce-gateway-stripe/9.2.0/includes/payment-methods/class-wc-stripe-payment-request.php on line 1475

On a regular store setup, this error should not happen since we deprecated PRBs in favor of express payment methods. But this change should at least make the code safer.

Testing instructions

Code review should be enough. Check if the tests are still passing.

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• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

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• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
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[wjrosa]

I just wrapped the code inside the try...catch block, but GitHub thinks I changed more stuff here.