Allow raven.dsn with "null" value

See: getsentry/raven-php@62701ba

Author: lyrixx

DependencyInjection/Configuration.php

@@ -263,7 +263,7 @@ public function getConfigTreeBuilder()
                             ->thenInvalid('The token and user have to be specified to use a PushoverHandler')
                         ->end()
                         ->validate()
-                            ->ifTrue(function($v) { return 'raven' === $v['type'] && !isset($v['dsn']); })
+                            ->ifTrue(function($v) { return 'raven' === $v['type'] && !array_key_exists('dsn', $v); })
                             ->thenInvalid('The DSN has to be specified to use a RavenHandler')
                         ->end()
                     ->end()