Fix warnings message and format code

plugin/buycourses/src/buy_course_plugin.class.php

@@ -1135,6 +1135,9 @@ class BuyCoursesPlugin extends Plugin
      */
     public function getNumInvoice($saleId, $isService) {
         $dataInvoice = $this->getDataInvoice($saleId, $isService);
+        if (empty($dataInvoice)) {
+            return '-';
+        }
 
         return $dataInvoice['serie'].$dataInvoice['year'].'/'.$dataInvoice['num_invoice'];
     }

plugin/buycourses/src/invoice.php

@@ -1,13 +1,11 @@
 <?php
 /* For license terms, see /license.txt */
-
+use Chamilo\CoreBundle\Component\Utils\ChamiloApi;
 /**
  * Print invoice of the Buy Courses plugin.
  *
  * @package chamilo.plugin.buycourses
  */
-use Chamilo\CoreBundle\Component\Utils\ChamiloApi;
-
 $cidReset = true;
 
 require_once '../config.php';