minor #11274 Added verbosity methods to NullOutput (WouterJ)

This PR was merged into the 2.4 branch.

Discussion
----------

Added verbosity methods to NullOutput

These 4 methods were not added to the OutputInterface because of BC, but they should still be implemented in all classes which implement that interface. Otherwise we have to do nasty tricks...

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
-------

0459249 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}