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/API/JSON/v1/NewsletterTemplatesTest.php

@@ -130,6 +130,6 @@ class NewsletterTemplatesTest extends \MailPoetTest {
     verify($response->status)->equals(APIResponse::STATUS_OK);
 
     $deletedTemplate = $this->newsletterTemplatesRepository->findOneById($templateId);
-    expect($deletedTemplate)->null();
+    verify($deletedTemplate)->null();
   }
 }