Reject requests without mandatory custom fields

We need to make sure subscribers cannot be created without custom fields
Users require GDPR consent and we need to make sure there are no way to
create a subscriber without mandatory custom fields

[MAILPOET-1405]

lib/API/MP/v1/API.php

@@ -6,6 +6,7 @@ use MailPoet\Models\Segment;
 use MailPoet\Models\Subscriber;
 use MailPoet\Models\SubscriberSegment;
 use MailPoet\Newsletter\Scheduler\Scheduler;
+use MailPoet\Subscribers\RequiredCustomFieldValidator;
 use MailPoet\Subscribers\Source;
 use MailPoet\Tasks\Sending;
 
@@ -164,6 +165,9 @@ class API {
     // if some required default fields are missing, set their values
     $default_fields = Subscriber::setRequiredFieldsDefaultValues($default_fields);
 
+    $validator = new RequiredCustomFieldValidator();
+    $validator->validate($custom_fields);
+
     // add subscriber
     $new_subscriber = Subscriber::create();
     $new_subscriber->hydrate($default_fields);