Added verbosity methods

src/Symfony/Component/Console/Output/NullOutput.php

@@ -74,6 +74,26 @@ class NullOutput implements OutputInterface
     {
         return self::VERBOSITY_QUIET;
     }
+    
+    public function isQuiet()
+    {
+        return true;
+    }
+
+    public function isVerbose()
+    {
+        return false;
+    }
+
+    public function isVeryVerbose()
+    {
+        return false;
+    }
+
+    public function isDebug()
+    {
+        return false;
+    }
 
     /**
      * {@inheritdoc}