Document Default env processor with null fallback

Author: jderusse

configuration/external_parameters.rst

@@ -533,6 +533,8 @@ Symfony provides the following env var processors:
             $container->setParameter('private_key', '%env(default:raw_key:file:PRIVATE_KEY)%');
             $container->setParameter('raw_key', '%env(PRIVATE_KEY)%');
 
+    When the fallback parameter is ommited (ie. ``env(default::API_KEY)``), the value retrieved is ``null``.
+
     .. versionadded:: 4.3
 
         The ``default`` processor was introduced in Symfony 4.3.