Fix front-end crash when no twig collector exists (#312)

If the twig collector wasn’t used on a particular request, the twig
widget crashes the client.  Fix the bug by assuming a default array of
zero templates.

(For example, an AJAX call that didn’t construct a TwigCollector would
trigger this crash.)

src/DebugBar/Bridge/Twig/TwigCollector.php

@@ -68,7 +68,7 @@ public function getWidgets()
  'icon' => 'leaf',
  'widget' => 'PhpDebugBar.Widgets.TemplatesWidget',
  'map' => 'twig',
- 'default' => '[]'
+ 'default' => json_encode(array('templates' => array())),
  ),
  'twig:badge' => array(
  'map' => 'twig.nb_templates',