Fixes zendframework#107 AbstractPluginManager plugin validation of empty serverice

Author: Martin Lonský

src/AbstractPluginManager.php

@@ -150,6 +150,9 @@ public function get($name, array $options = null)
      */
     public function validate($instance)
     {
+        if($instance === null){
+            return;
+        }
         if (method_exists($this, 'validatePlugin')) {
             trigger_error(sprintf(
                 '%s::validatePlugin() has been deprecated as of 3.0; please define validate() instead',