add a simple example with a null logger

NullObject/NullLogger.php

@@ -0,0 +1,38 @@
+<?php
+
+/*
+ * DesignPatternPHP
+ */
+
+namespace DesignPatterns\NullObject;
+
+/**
+ * NullOutput is a example of NullObject pattern. It is not formely a Design
+ * Pattern by the GoF but it's a schema which appears frequently enough to
+ * be a pattern. Futhermore it is a really good pattern in my opinion :
+ * - the code in the client is simple
+ * - it reduces the chance of null pointer exception
+ * - less "if" => less test cases
+ * 
+ * The purpose : every time you have a method which returns an object or null,
+ * you should return an object or a "NullObject". With NullObject, you don't need
+ * statement like "if (!is_null($obj)) { $obj->callSomething(); }" anymore.
+ *  
+ * In this case, this a logger which does nothing. Other examples :
+ * - null logger of symfony profiler
+ * - null output in symfony/console 
+ * - null handler in a Chain of Responsiblities pattern
+ * - null command in a Command pattern
+ * 
+ * Performance concerns : ok there is a call for nothing but we spare an "if is_null"
+ * I didn't run a benchmark but I think it's equivalent.
+ */
+class NullLogger implements LoggerInterface
+{
+
+    public function log($str)
+    {
+        
+    }
+
+}