javaunohelper: use a character literal

Change-Id: Ib0a69ac4714cd694ccd6a9bea87e32c1aadb0428
Reviewed-on: https://gerrit.libreoffice.org/11433
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>