Fix "Undefined index: time in environment_*.inc"

[`hook_cron_queue_info()`](https://api.drupal.org/api/drupal/modules!system!system.api.php/function/hook_cron_queue_info/7) declares that the `time` array element is optional, but `drush_queue_get_queues()` assumes that it's defined and therefore causes a PHP notice if it's not:

> Undefined index: time environment_7.inc:273 [notice]

This eliminates the assumption and eliminates the notice.