Fix PHP warnings

main/inc/lib/MoodleImport.php

@@ -24,14 +24,16 @@ class MoodleImport
 
         if (is_file($file) && is_readable($file)) {
             $package = new PclZip($file);
-            $packageContent = $package->listContent();
             $mainFileKey = 0;
+            $packageContent = $package->listContent();
+            if (!empty($packageContent)) {
                 foreach ($packageContent as $index => $value) {
                     if ($value['filename'] == 'moodle_backup.xml') {
                         $mainFileKey = $index;
                         break;
                     }
                 }
+            }
 
             if (!$mainFileKey) {
                 Display::addFlash(