scripting: redundant cast to String

Change-Id: I5e048964cc7885a32c59ff8a9eb1fc5b9523e843
Reviewed-on: https://gerrit.libreoffice.org/11859
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>