Add NOT NULL constraint to custom field value field type

lib/Config/Migrator.php

@@ -200,7 +200,7 @@ class Migrator {
       'id int(11) unsigned NOT NULL AUTO_INCREMENT,',
       'subscriber_id int(11) unsigned NOT NULL,',
       'custom_field_id int(11) unsigned NOT NULL,',
-      'value text,',
+      'value text NOT NULL,',
       'created_at TIMESTAMP NULL,',
       'updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,',
       'PRIMARY KEY  (id),',

tests/unit/Models/SubscriberTest.php

@@ -827,6 +827,7 @@ class SubscriberTest extends \MailPoetTest {
       $subscriber_custom_field->subscriber_id = ($custom_field !== 5) ?
         $subscriber->id :
         100; // create one record with a nonexistent subscriber id
+      $subscriber_custom_field->value = 'somevalue';
       $subscriber_custom_field->save();
     }
     expect(SubscriberCustomField::findMany())->count(5);