Fix easy errors found by PHPStan

[MAILPOET-2588]

lib/Models/ScheduledTask.php

@@ -126,6 +126,7 @@ class ScheduledTask extends Model {
       ORM::get_db()->rollBack();
       throw $error;
     }
+    return null;
   }
 
   function rescheduleProgressively() {

lib/Models/Subscriber.php

@@ -89,6 +89,7 @@ class Subscriber extends Model {
       SubscriberCustomField::deleteSubscriberRelations($this);
       return parent::delete();
     }
+    return null;
   }
 
   function trash() {