replaced Len() with isEmpty() where appropriate

Change-Id: I5a5847d6055f376fd5f86079aa1736541d790fff
Reviewed-on: https://gerrit.libreoffice.org/1892Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>