resolved fdo#61520 do not add multiple empty strings to the validation list

(cherry picked from commit 7122ef19847b26529ed1d5bad40df869e91a8495)

Backported.

Change-Id: I4f6180b2f8c914a965965ca362e1670e8b434d37
Reviewed-on: https://gerrit.libreoffice.org/8470
Tested-by: Kohei Yoshida <libreoffice@kohei.us>
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>