Making sure the plugin script directories variable is an array.

$pluginScriptDirs was giving me null sometimes, so I updated the check to make sure we have an array. Below was the error I was getting.

Warning: array_key_exists() expects parameter 2 to be array, null given in /var/www/html/digital_gallery/application/libraries/Omeka/Controller/Plugin/ViewScripts.php on line 155