Add validation to the set_payment_method_title_for_email method

[mdmoore]

Describe the bug

The set_payment_method_title_for_email function lacks proper validation, potentially leading to unexpected behavior or fatal errors when setting payment methods for email notifications. We should ensure that the function gracefully handles invalid or missing data and returns WooPayments as a fallback.

Additional context

Stack trace of one error that occurred during a pre-orders purchase and release

CRITICAL Uncaught WCPay\Exceptions\API_Exception: Error: Server error. Please try again. in /wp-content/plugins/woocommerce-payments/includes/wc-payment-api/class-wc-payments-api-client.php:2009
Stack trace:
#0 /wp-content/plugins/woocommerce-payments/includes/wc-payment-api/class-wc-payments-api-client.php(1835): WC_Payments_API_Client->check_response_for_errors(Array)
#1 /wp-content/plugins/woocommerce-payments/includes/wc-payment-api/class-wc-payments-api-client.php(1265): WC_Payments_API_Client->request(Array, 'payment_methods...', 'GET')
#2 /wp-content/plugins/woocommerce-payments/includes/payment-methods/class-upe-payment-gateway.php(1121): WC_Payments_API_Client->get_payment_method('')
#3 /wp-includes/class-wp-hook.php(310): WCPay\Payment_Methods\UPE_Payment_Gateway->set_payment_method_title_for_email(Object(WC_Order), false, false)
#4 /wp-content/plugins/woocommerce-payments/includes/wc-payment-api/class-wc-payments-api-client.php on line 2009

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