register an identity translator as fallback

The Form component can be used without the Translation component.
However, to be able to use the default form themes provided by the
TwigBridge you need to have the `trans` filter to be available.

This change ensure that there will always be a `trans` filter which as
a fallback will just return the message key if no translator is present.

Author: xabbuh

Extension/TranslationExtension.php

@@ -32,7 +32,7 @@ class TranslationExtension extends AbstractExtension
     private $translator;
     private $translationNodeVisitor;
 
-    public function __construct(TranslatorInterface $translator, NodeVisitorInterface $translationNodeVisitor = null)
+    public function __construct(TranslatorInterface $translator = null, NodeVisitorInterface $translationNodeVisitor = null)
     {
         if (!$translationNodeVisitor) {
             $translationNodeVisitor = new TranslationNodeVisitor();
@@ -94,11 +94,19 @@ public function getTranslationNodeVisitor()
 
     public function trans($message, array $arguments = array(), $domain = null, $locale = null)
     {
+        if (null === $this->translator) {
+            return $message;
+        }
+
         return $this->translator->trans($message, $arguments, $domain, $locale);
     }
 
     public function transchoice($message, $count, array $arguments = array(), $domain = null, $locale = null)
     {
+        if (null === $this->translator) {
+            return $message;
+        }
+
         return $this->translator->transChoice($message, $count, array_merge(array('%count%' => $count), $arguments), $domain, $locale);
     }