javaunohelper: warning: unchecked cast

Change-Id: I7c47e3a8657ec2368543088743879085878d78fb
Reviewed-on: https://gerrit.libreoffice.org/11444
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>