Replace expect()->isEmpty() with verify()->empty()

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

[MAILPOET-5664]

mailpoet/tests/integration/Models/SubscriberCustomFieldTest.php

@@ -64,7 +64,7 @@ class SubscriberCustomFieldTest extends \MailPoetTest {
       ]
     );
     $records = SubscriberCustomField::findArray();
-    expect($records)->isEmpty();
+    verify($records)->empty();
   }
 
   public function testItCanDeleteSubscriberRelations() {