fix the Deprecated functions explode() and boolean cast after introduction PHP 8

includes/destinations.ftp.inc

@@ -57,7 +57,9 @@ class backup_migrate_destination_ftp extends backup_migrate_destination_remote {
     $this->ftp_object();
     $ftp_files = (array) drupal_ftp_file_list('.', $this->ftp);
     foreach ($ftp_files as $file) {
-      $files[$file['filename']] = new backup_file($file);
+      if (isset($file['filename'])) {
+            $files[$file['filename']] = new backup_file($file);
+      }   
     }
     return $files;
   }