Check sidebar conditional tag exists

Return false for non-existent conditional tags. Fixes #993
What about this:

```php
  private function check_conditional_tag($conditional_tag) {
    list($tag_function, $args) = (array) $conditional_tag;

    if (function_exists($tag_function)) {
      return !empty($args) ? $tag_function($args) : $tag_function();
    } else {
      return false;
    }
  }
```