Restore null guard in get_config_for_payment_method

wc_payments_get_payment_gateway_by_id() can return false when a payment
method ID is not in the gateway map. Without the guard, accessing ->id
on false produces a PHP fatal error.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mgascam

includes/class-wc-payments-checkout.php

@@ -380,7 +380,10 @@ private function get_config_for_payment_method( $payment_method_id, $account_cou
 			'countries'      => $payment_method->get_countries(),
 		];
 
-		$gateway_for_payment_method    = $this->gateway->wc_payments_get_payment_gateway_by_id( $payment_method_id );
+		$gateway_for_payment_method = $this->gateway->wc_payments_get_payment_gateway_by_id( $payment_method_id );
+		if ( ! $gateway_for_payment_method ) {
+			return [];
+		}
 		$config['gatewayId']           = $gateway_for_payment_method->id;
 		$config['testingInstructions'] = WC_Payments_Utils::esc_interpolated_html(
 			/* translators: link to Stripe testing page */