Add missing unit test

Tests/Admin/BaseAdminTest.php

@@ -218,5 +218,7 @@ class BaseAdminTest extends \PHPUnit_Framework_TestCase
 
         $s = new FooTest_Admin;
         $this->assertEquals('salut', $admin->toString($s));
+
+        $this->assertEquals("", $admin->toString(false));
     }
 }