ZP-1119 Fixed warnings if device has no additional folders.

Released under the Affero GNU General Public License (AGPL) version 3.

src/lib/core/asdevice.php

@@ -810,8 +810,11 @@ class ASDevice extends StateObject {
      * @return array
      */
     public function GetAdditionalFolders() {
+        if (is_array($this->additionalfolders)) {
             return array_values($this->additionalfolders);
         }
+        return array();
+    }
 
     /**
      * Returns an additional folder by folder ID.