Replace expect()->null() with verify()->null()

codeception/verify 2.1 removed support for expect()->null() so we need
to replace it with verify()->null().

[MAILPOET-5664]

mailpoet/tests/integration/Cron/Workers/StatsNotifications/NewsletterLinkRepositoryTest.php

@@ -61,6 +61,6 @@ class NewsletterLinkRepositoryTest extends \MailPoetTest {
     $this->entityManager->flush();
 
     $nonExistingTopLink = $repository->findTopLinkForNewsletter((int)$newsletter2->getId());
-    expect($nonExistingTopLink)->null();
+    verify($nonExistingTopLink)->null();
   }
 }