Refactor customer email retrieval logic in booking and email handling

- Replace direct wash certificate email retrieval with `getWashCertificateEmail` method.
- Fallback to default email if no certificate-specific email exists.
- Adjust email assignment in booking notifications to use consistent logic.

services/nginx/app/classes/email.php

@@ -348,6 +348,7 @@ class email implements email_i
         $customer->requireSelected();
         $recipient_name = $customer->getCustomerName((int)$customer->customer_number->value());
         $recipient_email = $customer->email->value();
+        $customer_contact_email = ($recipient_email ?: '');
         // Check if the customer has a booking-specific email provided
         if (!empty($customer->wash_certificate_email->value())) {
             $recipient_email = (string)$customer->wash_certificate_email->value();
@@ -362,7 +363,7 @@ class email implements email_i
             $order_booking->id,
             $recipient_name,
             $customer->customer_number->value(),
-            $customer->email->value(),
+            $customer_contact_email,
             $order_booking->reference->value(),
             $order_booking->reg_1->value(),
             $order_booking->reg_2->value(),

services/nginx/app/objects/order_bookings_o.php

@@ -185,9 +185,10 @@ class order_bookings_o extends db
             // Check if the customer has a phone number
             $country_code = $customer->phone_country_code->value();
             $customer_phone = $customer->phone->value();
-            $customer_email = !empty($customer->wash_certificate_email->value())
-                ? $customer->wash_certificate_email->value()
-                : $customer->email->value();
+            $customer_email = $customer->getWashCertificateEmail();
+            if (!$customer_email) {
+                $customer_email = $customer->email->value();
+            }
             if (empty($customer_phone) && empty($customer_email)) {
                 return;
             }