Currently, the method eZ\Publish\Core\FieldType\Page\Value::__toString() does not always return String. It returns null if the page layout equals null. This can cause further issues because PHP throws an error if magic method __toString() returns something different then String.
Most of the other Value classes are casting the value returned from __toString() to String, so this is what most probably should also be done in this case.

Steps to reproduce

1. Set up new eZ Platform installation.
2. Create a new Symfony Command with the following code:

       <?php
   
   namespace AppBundle\Command;
   
   use eZ\Publish\Core\FieldType\Page\Parts\Page;
   use eZ\Publish\Core\FieldType\Page\Value;
   use Symfony\Bundle\FrameworkBundle\Command\ContainerAwareCommand;
   use Symfony\Component\Console\Input\InputInterface;
   use Symfony\Component\Console\Output\OutputInterface;
   
   class TestCommand extends ContainerAwareCommand
   {
       protected function configure()
       {
           $this
               ->setName('app:test_command');
       }
   
       protected function execute(InputInterface $input, OutputInterface $output)
       {
           $page = new Page();
           $pageValue = new Value($page);
   
           echo $pageValue;
       }
   }
   

3. Run the created command by executing the following:

   php app/console app:test_command
   

   You will be presented with the following error:

    [Symfony\Component\Debug\Exception\ContextErrorException]
     Catchable Fatal Error: Method eZ\Publish\Core\FieldType\Page\Value::__toString() must return a string value
   

   The expected behaviour is that the code runs without any error.