mobile apps need to have a 'files' array even empty

mobile.netbiblio.php

@@ -249,6 +249,8 @@ class NetBiblio extends WebService
         $files = json_decode($json, true);
         if (is_array($files)) {
             $book['files'] = $files;
+        } else {
+            $book['files'] = array();
         }
         return $book;
     }