Restore the constructor (empty) that I removed in d3e92fa4 because it can be...

Restore the constructor (empty) that I removed in d3e92fa4 because it can be called from subclasses

See https://github.com/piwik/piwik/commit/d3e92fa484956fd1edf853cac83e7d2c7a0f3310#commitcomment-10262413

core/Plugin/Menu.php

@@ -30,6 +30,11 @@ use Piwik\Plugins\UsersManager\UserPreferences;
  */
 class Menu
 {
+    public function __construct()
+    {
+        // Constructor kept for BC (because called in implementations)
+    }
+
     private function getModule()
     {
         $className = get_class($this);

core/Plugin/Widgets.php

@@ -25,6 +25,11 @@ class Widgets
     protected $category = '';
     protected $widgets  = array();
 
+    public function __construct()
+    {
+        // Constructor kept for BC (because called in implementations)
+    }
+
     /**
      * @ignore
      */