Allow setting services as null

src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php

@@ -220,6 +220,10 @@ class YamlFileLoader extends FileLoader
             return;
         }
 
+        if (null === $service) {
+            $service = array();
+        }
+
         if (!is_array($service)) {
             throw new InvalidArgumentException(sprintf('A service definition must be an array or a string starting with "@" but %s found for service "%s" in %s. Check your YAML syntax.', gettype($service), $id, $file));
         }

src/Symfony/Component/DependencyInjection/Tests/Fixtures/yaml/services28.yml

@@ -5,6 +5,8 @@ services:
         tags:
             - name: foo
 
+    Acme\Foo: ~
+
     with_defaults:
         class: Foo