foreach() error when building the Solr index

Whenever Solr indexes a node that is not within any queue, it causes a foreach() error.

# warning: Invalid argument supplied for foreach() in /var/www/sites/all/modules/contrib/nodequeue/nodequeue.module on line 358.

This patch makes Nodequeue consistently return an array in the nodequeue_get_subqueues_by_node() function, which makes sense that it would return an empty array if there aren't any subqueues.