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/Settings/UserFlagsControllerTest.php

@@ -82,7 +82,7 @@ class UserFlagsControllerTest extends \MailPoetTest {
       'userId' => $this->currentUserId,
       'name' => 'flag_1',
     ]);
-    expect($flag)->null();
+    verify($flag)->null();
   }
 
   private function createUserFlag($userId, $name, $value) {