yaml: use ~ instead of null

symfony/framework-bundle/3.3/config/packages/framework.yaml

@@ -1,9 +1,9 @@
 framework:
     secret: '%env(APP_SECRET)%'
     #default_locale: en
-    #csrf_protection: null
+    #csrf_protection: ~
     #http_method_override: true
-    #trusted_hosts: null
+    #trusted_hosts: ~
     # https://symfony.com/doc/current/reference/configuration/framework.html#handler-id
     #session:
     #    handler_id: session.handler.native_file

symfony/framework-bundle/3.3/config/packages/test/framework.yaml

@@ -1,4 +1,4 @@
 framework:
-    test: null
+    test: ~
     session:
         storage_id: session.storage.mock_file