removed unneeded implementation of Template::getTemplateName()

lib/Twig/Template.php

@@ -40,10 +40,7 @@ abstract class Twig_Template implements Twig_TemplateInterface
      *
      * @return string The template name
      */
-    public function getTemplateName()
-    {
-        return null;
-    }
+    abstract public function getTemplateName();
 
     /**
      * Returns the Twig environment.

test/Twig/Tests/TemplateTest.php

@@ -130,6 +130,10 @@ class Twig_Tests_TemplateTest extends PHPUnit_Framework_TestCase
 
 class Twig_TemplateTest extends Twig_Template
 {
+    public function getTemplateName()
+    {
+    }
+
     protected function doGetParent(array $context)
     {
     }