Fix a regression caused in #39

Author: mpdude

src/Caching/Attribute/Send304IfNotModified.php

@@ -28,6 +28,10 @@ public function __construct(...$values)
             \E_USER_DEPRECATED
         );
 
+        if (!$values) {
+            throw new \InvalidArgumentException(sprintf('The %s attribute needs at least one criterion', __CLASS__));
+        }
+
         $this->lastmodHelper = new LastmodHelper();
 
         foreach ($values as $key => $value) {

src/Caching/EventListener.php

@@ -109,7 +109,7 @@ protected function findAttribute($callback): ?Attribute\Send304IfNotModified
             $attributes = $method->getAttributes(\Webfactory\Bundle\WfdMetaBundle\Caching\Attribute\Send304IfNotModified::class);
 
             if ($attributes) {
-                return $attributes[0];
+                return $attributes[0]->newInstance();
             }
         }